Latest Research News
A study involving epilepsy patients who had electrodes implanted into their brain has revealed that memory consolidation during sleep doesn’t simply involve reactivation of the new memories.
Participants were given pictures to memorize, before taking an afternoon nap. Surprisingly, brainwave activity showed that both the pictures participants later remembered and those they later forgot, were reactivated during sleep. What was crucial was not the reactivation of the picture-specific gamma band activity, but its conjunction with “ripples” (extremely rapid fluctuations in activity) in the hippocampus. Only when the reactivation occurred at the same time as the ripples in the hippocampus did participants remember the picture.
What determined whether this happened? The evidence suggests that longer (and thus deeper) processing of the picture is needed, not simply a quick superficial look.
This phenomenon only occurred during nonREM sleep, not during wakefulness (the circumstances of sleep meant little time was spent in REM sleep).
The findings confirm earlier research with rodents.
Paper available at https://www.nature.com/articles/s41467-018-06553-y
(2018). Electrophysiological mechanisms of human memory consolidation.
Nature Communications. 9(1), 4103.
A laboratory study has found that sleeping after watching a trauma event reduced emotional distress and memories related to traumatic events. The laboratory study involved 65 women being shown a neutral and a traumatic video. Typically, recurring memories of certain images haunted the test subjects for a few days (these were recorded in detail in a diary). Some participants slept in the lab for a night after the video, while the other group remained awake.
Those who slept after the film had fewer and less distressing recurring emotional memories than those who were awake. This effect was particularly evident after several days.
One of the reasons for this benefit is thought to be that the memory consolidation processes that happen during sleep help contextualize the memories. This is interesting in view of the recent theory that PTSD is associated with a deficit in contextual processing.
However, I'd note that there is conflicting evidence about the effects of sleep on negative memories (for example, see http://www.memory-key.com/research/news/sleep-preserves-your-feelings-about-traumatic-events).
(2016). Effects of Sleep after Experimental Trauma on Intrusive Emotional Memories.
Sleep. 39(12), 2125 - 2132.
Sleep, as I have said on many occasions, helps your brain consolidate new memories. I have reported before on a number of studies showing how sleep helps the learning of various types of new information. Most of those studies have looked at procedural learning (learning new skills), or verbal learning. A new study adds to these by looking at face-name associations.
The small study, involving 14 young adults, found that that they were significantly better at remembering faces and names if they were given an opportunity to have a full night's sleep hours after seeing those faces and names for the first time.
Participants were shown 20 photos of faces with corresponding names and asked to memorize them. After a twelve-hour period, they were then shown the photos again with either a correct or incorrect name. They were also asked to rate their confidence in their answer. Each participant completed the test twice — once with an interval of sleep in between and once with a period of regular, waking day activities in between.
After a night's sleep, participants correctly matched 12% more of the faces and names, and were much more confident of their answers.
Of course, this is not a huge difference, given the small number of face-name pairs, and the sample is small. I would have also liked to see further testing 12 hours later, so that we could compare the effects of a day followed by a night, versus a night followed by a day (this would have required more stimuli and more participants, of course).
So, not madly exciting, but taken in context of other research, it adds to the growing evidence that sleep helps you consolidate new learning of all kinds.
(2015). A new face of sleep: The impact of post-learning sleep on recognition memory for face-name associations.
Neurobiology of Learning and Memory. 126, 31 - 38.
This sounds like pseudoscience, but it appears in Journal of Neuroscience, so … Weirdly, a rat study has found that sleeping on the side (the most common posture for humans and other animals) is the best position for efficiently removing waste from the brain.
Brain waste includes amyloid-beta and tau proteins, whose build-up is a critical factor in the development of Alzheimer's disease.
The study used imaging of the glymphatic pathway, which clears waste products from the brain by filtering cerebrospinal fluid through the brain and exchanging it with interstitial fluid. The process is most efficient during sleep, and its efficiency is affected by the level of consciousness. The researchers compared glymphatic transport during sleep when anesthetized rodents’ brains were in three positions—lateral (side), prone (down), and supine (up).
Of course, these findings need to be confirmed in humans (which might be tricky!), but there is, after all, no harm in changing your sleep position, if you don't already sleep on your side (though I concede it can be a difficult thing to change).
Apart from providing a practical tip for fighting age-related cognitive decline and dementia, the finding also supports the idea that one of the purposes of sleep is to ‘clean up’ the mess that accumulates while we are awake.
The finding is also consistent with increasing evidence that sleep disturbances are a factor in the development and progression of dementia.
(2015). The Effect of Body Posture on Brain Glymphatic Transport.
The Journal of Neuroscience. 35(31), 11034 - 11044.
We know sleep helps consolidate memories. Now a new study sheds light on how your sleeping brain decides what’s worth keeping. The study found that when the information that makes up a memory has a high value—associated with, for example, making more money—the memory is more likely to be rehearsed and consolidated during sleep.
The study involved 60 young adults who learned the unique locations of 72 objects on a screen while hearing characteristic object sounds. Each object was assigned a value indicating the reward that could be earned if remembered later. Recall was tested 45 minutes later, followed by a 90 minute break, during which participants either slept or remained awake. In the sleep condition, low-intensity white noise was played to mask any external sounds. In one condition, 18 of the sound cues associated with low-value objects were also repeatedly presented during the sleep period. In the wake condition, participants either watched a movie or performed a difficult working memory task (during which the sound cues were similarly sometimes presented in the background).
For all groups, at the first memory test, recall accuracy was significantly lower for low-value items compared to high-value (there was not, unsurprisingly, any difference between the groups). But let’s get to the important results. After sleep, in the absence of sound reminders, accuracy declined significantly more for low-value objects than for high-value objects. However, when sound cues had been played during sleep (although participants had no awareness of them), low-value objects were not differentially disadvantaged.
Interestingly, the sound reminders benefited not only those low-value objects which were cued, but all the low-value objects. But, in the wake condition, when sound cues had been softly played in the background, only those objects which had been cued benefited from the reminders.
Also interestingly, two participants who heard the cues during stage 2 sleep rather than slow-wave sleep received the least benefit.
What all this suggests is that covert reactivation may be a major factor in determining what gets chosen for consolidation, and wake and sleep reactivation might play distinct roles in this process - the former helping to strengthen individual, salient memories, and the latter strengthening, while also linking togther, categorically related memories.
The findings provide more weight to the idea that I have propounded before — that it’s worth consciously reviewing the day's memories that you want to keep, just before going to sleep.
(2013). The Role of Memory Reactivation during Wakefulness and Sleep in Determining Which Memories Endure.
The Journal of Neuroscience. 33(15), 6672 - 6678.
Recent research has suggested that sleep problems might be a risk factor in developing Alzheimer’s, and in mild cognitive impairment. A new study adds to this gathering evidence by connecting reduced slow-wave sleep in older adults to brain atrophy and poorer learning.
The study involved 18 healthy young adults (mostly in their 20s) and 15 healthy older adults (mostly in their 70s). Participants learned 120 word- nonsense word pairs and were tested for recognition before going to bed. Their brain activity was recorded while they slept. Brain activity was also measured in the morning, when they were tested again on the word pairs.
As has been found previously, older adults showed markedly less slow-wave activity (both over the whole brain and specifically in the prefrontal cortex) than the younger adults. Again, as in previous studies, the biggest difference between young and older adults in terms of gray matter volume was found in the medial prefrontal cortex (mPFC). Moreover, significant differences were also found in the insula and posterior cingulate cortex. These regions, like the mPFC, have also been associated with the generation of slow waves.
When mPFC volume was taken into account, age no longer significantly predicted the extent of the decline in slow-wave activity — in other words, the decline in slow-wave activity appears to be due to the brain atrophy in the medial prefrontal cortex. Atrophy in other regions of the brain (precuneus, hippocampus, temporal lobe) was not associated with the decline in slow-wave activity when age was considered.
Older adults did significantly worse on the delayed recognition test than young adults. Performance on the immediate test did not predict performance on the delayed test. Moreover, the highest performers on the immediate test among the older adults performed at the same level as the lowest young adult performers — nevertheless, these older adults did worse the following day.
Slow-wave activity during sleep was significantly associated with performance on the next day’s test. Moreover, when slow-wave activity was taken into account, neither age nor mPFC atrophy significantly predicted test performance.
In other words, age relates to shrinkage of the prefrontal cortex, this shrinkage relates to a decline in slow-wave activity during sleep, and this decline in slow-wave sleep relates to poorer cognitive performance.
The findings confirm the importance of slow-wave brainwaves for memory consolidation.
All of this suggests that poorer sleep quality contributes significantly to age-related cognitive decline, and that efforts should be made to improve quality of sleep rather than just assuming lighter, more disturbed sleep is ‘natural’ in old age!
(2013). Prefrontal atrophy, disrupted NREM slow waves and impaired hippocampal-dependent memory in aging.
Because sleep is so important for memory and learning (and gathering evidence suggests sleep problems may play a significant role in age-related cognitive impairment), I thought I’d make quick note of a recent review bringing together all research on the immediate effects of alcohol on the sleep of healthy individuals.
The review found that alcohol in any amount reduces the time it takes to fall asleep, while greater amounts produce increasing amounts of deep sleep in the first half of the night. However, sleep is more disrupted in the second half. While increased deep sleep is generally good, there are two down sides here: first, it’s paired with sleep disruption in the second half of the night; second, those predisposed to problems such as sleepwalking or sleep apnea may be more vulnerable to them. (A comment from the researchers that makes me wonder if the relationship between deep sleep and slow-wave activity is more complicated than I realized.)
Additionally, at high doses of alcohol, REM sleep is significantly reduced in the first half, and overall. This may impair attention, memory, and motor skills. Moreover, at all doses, the first REM period is significantly delayed, producing less restful sleep.
The researchers conclude that, while alcohol may give the illusion of improving sleep, it is not in fact doing so.
(2013). Alcohol and Sleep I: Effects on Normal Sleep.
Alcoholism: Clinical and Experimental Research. n/a - n/a.
Back in 2010, I briefly reported on a study suggesting that a few minutes of ‘quiet time’ could help you consolidate new information. A new study provides more support for this idea.
In the first experiment, 14 older adults (aged 61-81) were told a short story, with instructions to remember as many details as possible. Immediately afterward, they were asked to describe what happened in the story. Ten minutes then elapsed, during which they either rested quietly (with eyes closed in a darkened room), or played a spot-the-difference game on the computer (comparing pairs of pictures). This task was chosen because it was non-verbal and sufficiently different from the story task to not directly compete for cognitive resources.
This first learning phase was followed by five minutes of playing the spot-the-difference game (for all participants) and then a second learning phase, in which the process was repeated with a second story, and participants experienced the other activity during the delay period (e.g., rest if they had previously played the game).
Some 30 minutes after the first story presentation (15 minutes after the second), participants were unexpectedly asked to once again recall as many details as they could from the stories. A further recall test was also given one week later.
Recall on the first delayed test (at the end of both learning phases) was significantly better for stories that had been followed by wakeful resting rather than a game. While recall declined at the same rate for both story conditions, the benefits of wakeful resting were maintained at the test one week later.
In a second experiment, the researchers looked at whether these benefits would still occur if there was no repetition (i.e., no delayed recall test at the time, only at a week). Nineteen older adults (61-87) participated.
As expected, in the absence of the short-delay retrieval test, recall at a week was slightly diminished. Nevertheless, recall for stories that had been followed by rest was still significantly better than recall for stories followed by the game.
It’s worth noting that, in a post-session interview, only 3 participants (of the 33 total) reported thinking about the story during the period of wakeful rest. One participant fell asleep. Twelve participants reported thinking about the stories at least once during the week, but there was no difference between these participants’ scores and those who didn’t think about them.
These findings support the idea that a quiet period of reflection after new learning helps the memories be consolidated. While the absence of interfering information may underlie this, the researchers did select the game specifically to interfere as little as possible with the story task. Moreover, the use of the same task as a ‘filler’ between the two learning phases was also designed to equalize any interference it might engender.
The weight of the evidence, therefore, is that ten minutes of wakeful resting aided memory by providing the mental space in which to consolidate the memory. Moreover, the fact that so few participants actively thought about the stories during that rest indicates that such consolidation is automatic and doesn’t require deliberate rehearsal.
The study did, of course, only involve older adults. I hope we will see a larger study with a wider participant pool.
(2012). Brief Wakeful Resting Boosts New Memories Over the Long Term.
Back when I was young, sleep learning was a popular idea. The idea was that a tape would play while you were asleep, and learning would seep into your brain effortlessly. It was particularly advocated for language learning. Subsequent research, unfortunately, rejected the idea, and gradually it has faded (although not completely). Now a new study may presage a come-back.
In the study, 16 young adults (mean age 21) learned how to ‘play’ two artificially-generated tunes by pressing four keys in time with repeating 12-item sequences of moving circles — the idea being to mimic the sort of sensorimotor integration that occurs when musicians learn to play music. They then took a 90-minute nap. During slow-wave sleep, one of the tunes was repeatedly played to them (20 times over four minutes). After the nap, participants were tested on their ability to play the tunes.
A separate group of 16 students experienced the same events, but without the playing of the tune during sleep. A third group stayed awake, during which 90-minute period they played a demanding working memory task. White noise was played in the background, and the melody was covertly embedded into it.
Consistent with the idea that sleep is particularly helpful for sensorimotor integration, and that reinstating information during sleep produces reactivation of those memories, the sequence ‘practiced’ during slow-wave sleep was remembered better than the unpracticed one. Moreover, the amount of improvement was positively correlated with the proportion of time spent in slow-wave sleep.
Among those who didn’t hear any sounds during sleep, improvement likewise correlated with the proportion of time spent in slow-wave sleep. The level of improvement for this group was intermediate to that of the practiced and unpracticed tunes in the sleep-learning group.
The findings add to growing evidence of the role of slow-wave sleep in memory consolidation. Whether the benefits for this very specific skill extend to other domains (such as language learning) remains to be seen.
However, another recent study carried out a similar procedure with object-location associations. Fifty everyday objects were associated with particular locations on a computer screen, and presented at the same time with characteristic sounds (e.g., a cat with a meow and a kettle with a whistle). The associations were learned to criterion, before participants slept for 2 hours in a MR scanner. During slow-wave sleep, auditory cues related to half the learned associations were played, as well as ‘control’ sounds that had not been played previously. Participants were tested after a short break and a shower.
A difference in brain activity was found for associated sounds and control sounds — associated sounds produced increased activation in the right parahippocampal cortex — demonstrating that even in deep sleep some sort of differential processing was going on. This region overlapped with the area involved in retrieval of the associations during the earlier, end-of-training test. Moreover, when the associated sounds were played during sleep, parahippocampal connectivity with the visual-processing regions increased.
All of this suggests that, indeed, memories are being reactivated during slow-wave sleep.
Additionally, brain activity in certain regions at the time of reactivation (mediotemporal lobe, thalamus, and cerebellum) was associated with better performance on the delayed test. That is, those who had greater activity in these regions when the associated sounds were played during slow-wave sleep remembered the associations best.
The researchers suggest that successful reactivation of memories depends on responses in the thalamus, which if activated feeds forward into the mediotemporal lobe, reinstating the memories and starting the consolidation process. The role of the cerebellum may have to do with the procedural skill component.
The findings are consistent with other research.
All of this is very exciting, but of course this is not a strategy for learning without effort! You still have to do your conscious, attentive learning. But these findings suggest that we can increase our chances of consolidating the material by replaying it during sleep. Of course, there are two practical problems with this: the material needs an auditory component, and you somehow have to replay it at the right time in your sleep cycle.
(2012). Cued memory reactivation during sleep influences skill learning.
(2012). Memory stabilization with targeted reactivation during human slow-wave sleep.
Proceedings of the National Academy of Sciences. 109(26), 10575 - 10580.
We know that we remember more 12 hours after learning if we have slept during that 12 hours rather than been awake throughout, but is this because sleep is actively helping us remember, or because being awake makes it harder to remember (because of interference and over-writing from other experiences). A new study aimed to disentangle these effects.
In the study, 207 students were randomly assigned to study 40 related or unrelated word pairs at 9 a.m. or 9 p.m., returning for testing either 30 minutes, 12 hours or 24 hours later.
As expected, at the 12-hour retest, those who had had a night’s sleep (Evening group) remembered more than those who had spent the 12 hours awake (Morning group). But this result was because memory for unrelated word pairs had deteriorated badly during 12 hours of wakefulness; performance on the related pairs was the same for the two groups. Performance on the related and unrelated pairs was the same for those who slept.
For those tested at 24 hours (participants from both groups having received both a full night of sleep and a full day of wakefulness), those in the Evening group (who had slept before experiencing a full day’s wakefulness) remembered significantly more than the Morning group. Specifically, the Evening group showed a very slight improvement over training, while the Morning group showed a pronounced deterioration.
This time, both groups showed a difference for related versus unrelated pairs: the Evening group showed some deterioration for unrelated pairs and a slightly larger improvement for related pairs; the Morning group showed a very small deterioration for related pairs and a much greater one for unrelated pairs. The difference between recall of related pairs and recall of unrelated pairs was, however, about the same for both groups.
In other words, unrelated pairs are just that much harder to learn than related ones (which we already know) — over time, learning them just before sleep vs learning early in the day doesn’t make any difference to that essential truth. But the former strategy will produce better learning for both types of information.
A comparison of the 12-hour and 24-hour results (this is the bit that will help us disentangle the effects of sleep and wakefulness) reveals that twice as much forgetting of unrelated pairs occurred during wakefulness in the first 12 hours, compared to wakefulness in the second 12 hours (after sleep), and 3.4 times more forgetting of related pairs (although this didn’t reach significance, the amount of forgetting being so much smaller).
In other words, sleep appears to slow the rate of forgetting that will occur when you are next awake; it stabilizes and thus protects the memories. But the amount of forgetting that occurred during sleep was the same for both word types, and the same whether that sleep occurred in the first 12 hours or the second.
Participants in the Morning and Evening groups took a similar number of training trials to reach criterion (60% correct), and there was no difference in the time it took to learn unrelated compared to related word pairs.
It’s worth noting that there was no difference between the two groups, or for the type of word pair, at the 30-minutes test either. In other words, your ability to remember something shortly after learning it is not a good guide for whether you have learned it ‘properly’, i.e., as an enduring memory.
The study tells us that the different types of information are differentially affected by wakefulness, that is, perhaps, they are more easily interfered with. This is encouraging, because semantically related information is far more common than unrelated information! But this may well serve as a reminder that integrating new material — making sure it is well understood and embedded into your existing database — is vital for effective learning.
The findings also confirm earlier evidence that running through any information (or skills) you want to learn just before going to bed is a good idea — and this is especially true if you are trying to learn information that is more arbitrary or less well understood (i.e., the sort of information for which you are likely to use mnemonic strategies, or, horror of horrors, rote repetition).
(2012). Memory for Semantically Related and Unrelated Declarative Information: The Benefit of Sleep, the Cost of Wake.
PLoS ONE. 7(3), e33079 - e33079.
Full text available at http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0033079
Previous research has shown that negative objects and events are preferentially consolidated in sleep — if you experience them in the evening, you are more likely to remember them than more neutral objects or events, but if you experience them in the morning, they are not more likely to be remembered than other memories (see collected sleep reports). However, more recent studies have failed to find this. A new study also fails to find such preferential consolidation, but does find that our emotional reaction to traumatic or disturbing events can be greatly reduced if we stay awake afterward.
Being unable to sleep after such events is of course a common response — these findings indicate there’s good reason for it, and we should go along with it rather than fighting it.
The study involved 106 young adults rating pictures on a sad-happy scale and their own responses on an excited-calm scale. Twelve hours later, they were given a recognition test: noting pictures they had seen earlier from a mix of new and old pictures. They also rated all the pictures on the two scales. There were four groups: 41 participants saw the first set late in the day and the second set 12 hours later on the following day (‘sleep group’); 41 saw the first set early and the second set 12 hours later on the same day; 12 participants saw both sets in the evening, with only 45 minutes between the sets; 12 participants saw both sets in the morning (these last two groups were to rule out circadian effects). 25 of the sleep group had their brain activity monitored while they slept.
The sleep group performed significantly better on the recognition test than the same-day group. Negative pictures were remembered better than neutral ones. However, unlike earlier studies, the sleep group didn’t preferentially remember negative pictures more than the same-day group.
But, interestingly, the sleep group was more likely to maintain the strength of initial negative responses. The same-day group showed a weaker response to negative scenes on the second showing.
It’s been theorized that late-night REM sleep is critical for emotional memory consolidation. However, this study found no significant relationship between the amount of time spent in REM sleep and recognition memory, nor was there any relationship between other sleep stages and memory. There was one significant result: those who had more REM sleep in the third quarter of the night showed the least reduction of emotional response to the negative pictures.
There were no significant circadian effects, but it’s worth noting that even the 45 minute gap between the sets was sufficient to weaken the negative effect of negative scenes.
While there was a trend toward a gender effect, it didn’t reach statistical significance, and there were no significant interactions between gender and group or emotional value.
The findings suggest that the effects of sleep on memory and emotion may be independent.
The findings also contradict previous studies showing preferential consolidation of emotional memories during sleep, but are consistent with two other recent studies that have also failed to find this. At this stage, all we can say is that there may be certain conditions in which this occurs (or doesn’t occur), but more research is needed to determine what these conditions are. Bear in mind that there is no doubt that sleep helps consolidate memories; we are talking here only about emphasizing negative memories at the expense of emotionally-neutral ones.
(2012). Processing of Emotional Reactivity and Emotional Memory over Sleep.
The Journal of Neuroscience. 32(3), 1035 - 1042.
Sleep can boost classroom performance of college students
There’s a lot of evidence that memories are consolidated during sleep, but most of it has involved skill learning. A new study extends the findings to complex declarative information — specifically, information from a lecture on microeconomics.
The study involved 102 university undergraduates who had never taken an economics course. In the morning or evening they completed an introductory, virtual lecture that taught them about concepts and problems related to supply and demand microeconomics. They were then tested on the material either immediately, after a 12-hour period that included sleep, after 12 hours without sleep, or after one week. The test included both basic problems that they had been trained to solve, and "transfer" problems that required them to extend their knowledge to novel, but related, problems.
Performance was better for those who slept, and this was especially so for the novel, 'transfer' integration problems.
Rule-learning task also benefits from sleep
Another complex cognitive task was investigated in a study of 54 college undergraduates who were taught to play a card game for rewards of play money in which wins and losses for various card decks mimic casino gambling (the Iowa Gambling Task is typically used to assess frontal lobe function). Those who had a normal night’s sleep as part of the study drew from decks that gave them the greatest winnings four times more often than those who spent the 12-hour break awake, and they better understood the underlying rules of the game.
The students were given a brief morning or afternoon preview of the gambling task (too brief to learn the underlying rule). They returned twelve hours later (i.e., either after a normal night’s sleep, or after a day of their usual activities), when they played the full gambling task for long enough to learn the rules. Those who got to sleep between the two sessions played better and showed a better understanding of the rules when questioned.
To assure that time of day didn’t explain the different performance, two groups of 17 and 21 subjects carried out both the preview and the full task either in the morning or the evening. Time of day made no difference.
Sleep problems may be a link between perceived racism and poor health
Analysis of data from the 2006 Behavioral Risk Factor Surveillance System, involving 7,093 people in Michigan and Wisconsin, suggests that sleep deprivation may be one mediator of the oft-reported association between discrimination and poorer cognitive performance.
The survey asked the question: "Within the past 12 months when seeking health care, do you feel your experiences were worse than, the same as, or better than for people of other races?" Taking this as an index of perceived racism, and comparing it with reports of sleep disturbance (difficulty sleeping at least six nights in the past two weeks), the study found that individuals who perceived racial discrimination were significantly more likely to experience sleep difficulties, even after allowing for socioeconomic factors and depression. Risk of sleep disturbance was nearly doubled in those who perceived themselves as discriminated against, and although this was reduced after depression was taken into account, it remained significant.
Sleep problems more prevalent than expected in urban minority children
Ten families also underwent sleep monitoring at home for five to seven days. All children who completed actigraphy monitoring had shortened sleep duration, with an average sleep duration of 8 hours, significantly less than the 10 to 11 hours recommended for children in this age group.
It’s worth noting that parents consistently overestimated sleep duration. Although very aware of bedtime and wake time, parents are less aware of time spent awake during the night.
(Also note that the figures I quote are taken from the conference abstract, which differ from those quoted in the press release.)
Rocking really does help sleep
If you or your loved one is having troubles getting to sleep, you might like to note an intriguing little study involving 12 healthy males (aged 22-38, and good sleepers). The men twice took a 45-minute afternoon nap on a bed that could slowly rock. On one occasion, it was still; on the other, it rocked. Rocking brought about faster sleep, faster transition to deeper sleep, and increased slow oscillations and sleep spindles (hallmarks of deep sleep). All these results were evident in every participant.
Sleep helps long-term memory in two ways
A fruit fly study points to two dominant theories of sleep being correct. The two theories are (a) that synapses are pruned during sleep, ensuring that only the strongest connections survive (synaptic homeostasis), and (b) that memories are replayed and consolidated during sleep, so that some connections are reactivated and thus made stronger (memory consolidation).
The experiment was made possible by the development of a new strain of fruit fly that can be induced to fall asleep when temperatures rise. The synaptic homeostasis model was supported when flies were placed in socially enriched environments, then either induced to sleep or not, before being taught a courtship ritual. Those that slept developed long-term memories of the ritual, while those that didn’t sleep didn’t remember it. The memory consolidation theory was supported when flies trained using a protocol designed to give them short-term memories retained a lasting memory, if sleep was induced immediately after the training.
In other words, it seems that both pruning and replaying are important for building long-term memories.
Mouse studies identify the roots of memory impairment resulting from sleep deprivation
Sleep deprivation in known to result in increased levels of adenosine in the brain, whether fruit fly or human (caffeine blocks the effects of adenosine). New mice studies now reveal the mechanism.
Mice given a drug that blocked a particular adenosine receptor in the hippocampus (the A1 receptor) failed to show the normal memory impairment evoked by sleep deprivation (being woken halfway through their normal 12-hour sleep schedule). The same results occurred if mice were genetically engineered to lack a gene involved in the production of glial transmitters (necessary to produce adenosine).
Memory was tested by the mice being allowed to explore a box with two objects, and then returned to the box on the next day, where one of the two objects had been moved. They would normally explore the moved object more than other objects, but sleep-deprived mice don’t usually react to the change, because they don’t remember where the object had been. In both these cases, the sleep-deprived mice showed no memory impairment.
Both the drugged and genetically protected mice also showed greater synaptic plasticity in the hippocampus after being sleep deprived than the untreated group.
The two groups reveal two parts of the chemical pathway involved in sleep deprivation. The genetic engineering experiment shows that the adenosine comes from glia's release of adenosine triphosphate (ATP). The drug experiment shows that the adenosine goes to the A1 receptor in the hippocampus.
The findings provide the first evidence that astrocytic ATP and adenosine A1R activity contribute to the effects of sleep deprivation on hippocampal synaptic plasticity and hippocampus-dependent memory, and suggest a new therapeutic target to reverse the cognitive deficits induced by sleep loss.
Scullin M, McDaniel M, Howard D, Kudelka C. 2011. Sleep and testing promote conceptual learning of classroom materials. Presented Tuesday, June 14, in Minneapolis, Minn., at SLEEP 2011, the 25th Anniversary Meeting of the Associated Professional Sleep Societies LLC (APSS).
(Submitted). Sleep‐dependent modulation of affectively guided decision‐making.
Journal of Sleep Research.
Grandner MA, Hale L, Jackson NJ, Patel NP, Gooneratne N, Troxel WM. 2011. Sleep disturbance and daytime fatigue associated with perceived racial discrimination. Presented Tuesday, June 14, in Minneapolis, Minn., at SLEEP 2011, the 25th Anniversary Meeting of the Associated Professional Sleep Societies LLC (APSS).
Sheares, B.J., Dorsey, K.B., Lamm, C.I., Wei, Y., Kattan, M., Mellins, R.B. & Evans, D. 2011. Sleep Problems In Urban Minority Children May Be More Prevalent Than Previously Recognized. Presented at the ATS 2011 International Conference in Denver.
(2011). Rocking synchronizes brain waves during a short nap.
Current Biology. 21(12), R461-R462 - R461-R462.
(2011). Inducing Sleep by Remote Control Facilitates Memory Consolidation in Drosophila.
Science. 332(6037), 1571 - 1576.
(2011). Astrocyte-Derived Adenosine and A1 Receptor Activity Contribute to Sleep Loss-Induced Deficits in Hippocampal Synaptic Plasticity and Memory in Mice.
The Journal of Neuroscience. 31(19), 6956 - 6962.
Sleep can boost classroom performance of college students http://www.eurekalert.org/pub_releases/2011-06/aaos-scb060611.php Rule-learning task also benefits from sleep http://medicalxpress.com/news/2011-05-excellent-science-based-advice.html Sleep problems may be a link between perceived racism and poor health http://medicalxpress.com/news/2011-06-problems-link-racism-poor-health.html Sleep problems more prevalent than expected in urban minority children http://medicalxpress.com/news/2011-05-problems-prevalent-urban-minority-... Rocking really does help sleep http://www.scientificamerican.com/podcast/episode.cfm?id=rocking-increas... Sleep helps long-term memory in two ways http://the-scientist.com/2011/06/23/sleep-on-it/ Mouse studies identify the roots of memory impairment resulting from sleep deprivation http://www.eurekalert.org/pub_releases/2011-05/uop-pri051711.php
In a study involving 44 young adults given a rigorous memorizing task at noon and another such task at 6pm, those who took a 90-minute nap during the interval improved their ability to learn on the later task, while those who stayed awake found it harder to learn.
The degree to which the nappers were refreshed correlated with the amount of stage 2 non-REM sleep they experienced. This sleep phase is characterized by sleep spindles, which are associated with brain activity between the hippocampus and prefrontal cortex. Spindle-rich sleep occurs mostly in the second half of the night, so those who don’t get their quota of sleep are probably getting less.
The finding confirms the idea that learning ability decreases the more time you spend awake.
(2011). Wake deterioration and sleep restoration of human learning.
Current Biology. 21(5), R183-R184 - R183-R184.
Two experiments involving a total of 191 volunteers have investigated the parameters of sleep’s effect on learning. In the first experiment, people learned 40 pairs of words, while in the second experiment, subjects played a card game matching pictures of animals and objects, and also practiced sequences of finger taps. In both groups, half the volunteers were told immediately following the tasks that they would be tested in 10 hours. Some of the participants slept during this time.
As expected, those that slept performed better on the tests (all of them: word recall, visuospatial, and procedural motor memory), but the really interesting bit is that it turned out it was only the people who slept who also knew a test was coming that had improved memory recall. These people showed greater brain activity during deep or "slow wave" sleep, and for these people only, the greater the activity during slow-wave sleep, the better their recall.
Those who didn’t sleep, however, were unaffected by whether they knew there would be a test or not.
Of course, this doesn’t mean you never remember things you don’t intend or want to remember! There is more than one process going on in the encoding and storing of our memories. However, it does confirm the importance of intention, and cast light perhaps on some of your learning failures.
(2011). Sleep Selectively Enhances Memory Expected to Be of Future Relevance.
The Journal of Neuroscience. 31(5), 1563 - 1569.
The role of sleep in consolidating memory is now well-established, but recent research suggests that sleep also reorganizes memories, picking out the emotional details and reconfiguring the memories to help you produce new and creative ideas. In an experiment in which participants were shown scenes of negative or neutral objects at either 9am or 9pm and tested 12 hours later, those tested on the same day tended to forget the negative scenes entirely, while those who had a night’s sleep tended to remember the negative objects but not their neutral backgrounds.
Follow-up experiments showed the same selective consolidation of emotional elements to a lesser degree after a 90-minute daytime nap, and to a greater degree after a 24-hour or even several-month delay (as long as sleep directly followed encoding).
These findings suggest that processes that occur during sleep increase the likelihood that our emotional responses to experiences will become central to our memories of them. Moreover, additional nights of sleep may continue to modify the memory.
In a different approach, another recent study has found that when volunteers were taught new words in the evening, then tested immediately, before spending the night in the sleep lab and being retested in the morning, they could remember more words in the morning than they did immediately after learning them, and they could recognize them faster. In comparison, a control group who were trained in the morning and re-tested in the evening showed no such improvement on the second test.
Deep sleep (slow-wave sleep) rather than rapid eye movement (REM) sleep or light sleep appeared to be the important phase for strengthening the new memories. Moreover, those who experienced more sleep spindles overnight were more successful in connecting the new words to the rest of the words in their mental lexicon, suggesting that the new words were communicated from the hippocampus to the neocortex during sleep. Sleep spindles are brief but intense bursts of brain activity that reflect information transfer between the hippocampus and the neocortex.
The findings confirm the role of sleep in reorganizing new memories, and demonstrate the importance of spindle activity in the process.
Taken together, these studies point to sleep being more important to memory than has been thought. The past decade has seen a wealth of studies establishing the role of sleep in consolidating procedural (skill) memory, but these findings demonstrate a deeper, wider, and more ongoing process. The findings also emphasize the malleability of memory, and the extent to which they are constructed (not copied) and reconstructed.
(2010). Sleep’s Role in the Consolidation of Emotional Episodic Memories.
Current Directions in Psychological Science. 19(5), 290 - 295.
(2010). Sleep Spindle Activity is Associated with the Integration of New Memories and Existing Knowledge.
J. Neurosci.. 30(43), 14356 - 14360.
It is now well established that memories are consolidated during sleep. Now a new study has found that restful periods while you are awake are also times when consolidation can occur. The imaging study revealed that during resting (allowed to think about anything), there was correlated activity between the hippocampus and part of the lateral occipital complex. This activity was associated with improved memory for the previous experience. Moreover, the degree of activity correlated with how well it was remembered. You can watch a 4 ½ minute video where the researchers explain their study at http://www.cell.com/neuron/abstract/S0896-6273%2810%2900006-1
Tambini, A., Ketz, N. & Davach, L. 2010. Enhanced Brain Correlations during Rest Are Related to Memory for Recent Experiences. Neuron, 65 (2), 280-290.
Following on from research showing that pulling an all-nighter decreases the ability to cram in new facts by nearly 40%, a study involving 39 young adults has found that those given a 90-minute nap in the early afternoon, after being subjected to a rigorous learning task, did markedly better at a later round of learning exercises, compared to those who remained awake throughout the day. The former group actually improved in their capacity to learn, while the latter became worse at learning. The findings reinforce the hypothesis that sleep is needed to clear the brain's short-term memory storage and make room for new information. Moreover, this refreshing of memory capacity was related to Stage 2 non-REM sleep (an intermediate stage between deep sleep and the REM dream stage).
The preliminary findings were presented February 21, at the annual meeting of the American Association of the Advancement of Science (AAAS) in San Diego, Calif.
A number of studies have shown the benefits of sleep for consolidating motor learning. A new study extends this research to a more complex motor task: "Guitar Hero III", a popular video game. There was significantly greater improvement after a night’s sleep (average 68% in performance accuracy vs 63% for students who learnt the task in the morning and were tested in the evening), and a significant correlation between sleep duration and the amount of improvement.
Higginson, C.D. et al. 2010. So you wanna be a rock star? Sleep on it. Presented at SLEEP 2010, the 24th annual meeting of the Associated Professional Sleep Societies LLC, in San Antonio, Texas.
It’s now well established that sleep plays an important role in memory and learning. Now a new study suggests that dreams also play a part in consolidating memories. The study involved 99 subjects training for an hour on a computerized maze task, and then either taking a 90-minute nap or engaging in quiet activities. Intermittently, subjects were asked to describe what was going through their minds, or what they had been dreaming about. Five hours after training, the subjects were retested on the maze task. While those who hadn’t slept showed no improvement on the second test (even if they had reported thinking about the maze during their rest period), and those nappers who reported no maze-related dreams also showed little improvement, those who dreamed about the task showed dramatic improvement. Those who dreamed about the task were not more interested or motivated, but they were more likely to have performed relatively poorly during training — suggesting that the sleeping brain is more likely to focus on areas of greatest need. The researchers believe not that dreaming causes you to remember, but that dreaming is a marker that the brain is working on a problem at many levels — perhaps reflecting the brain's attempt to find useful associations.
(2010). Dreaming of a Learning Task Is Associated with Enhanced Sleep-Dependent Memory Consolidation.
Current Biology. 20(9), 850 - 855.
In this study, subjects were shown two sets of 12 color photographs of people’s faces (24 in total). Five minutes after seeing the last one, the subjects were then shown another 48 faces (one by one, as before) and had to say whether or not they had seen the face earlier. If so, they were asked whether they saw it in the first or second set of photographs. Half the subjects had been deprived of sleep for the previous 35 hours. Some of these had been given significant amounts of caffeine to offset their sleepiness.
It was found that the sleep-deprived subjects, whether or not they had had caffeine, were as good as the non-sleep-deprived subjects at recognizing which faces they had seen before. However, the sleep-deprived subjects were significantly worse at remembering which set the faces had appeared in. This occurred even though otherwise optimum conditions for recall existed (the test was novel, stimulating, and relatively short; it was given at the best time of day for maximum alertness).
Caffeine significantly reduced the feelings of sleepiness and did appear to improve the ability of the sleep-deprived subjects to remember which set the face had appeared in, but the level of recall was still significantly below the level of the non-sleep-deprived subjects. Caffeine made no difference to the memory performance of subjects who were not sleep-deprived.
Interestingly, sleep deprivation increased the subjects’ belief that they were right, especially when they were wrong. In this case, whether or not they had had caffeine made no difference.
It may be that the problem with temporal memory reflects a more general problem with remembering context information.
Harrison, Yvonne & Horne, James A. 2000. Sleep loss and temporal memory. The Quarterly Journal of Experimental Psychology, 53A (1), 271-279.
Older news items (pre-2010) brought over from the old website
A midday nap markedly boosts the brain's learning capacity
Following on from research showing that pulling an all-nighter decreases the ability to cram in new facts by nearly 40%, a study involving 39 young adults has found that those given a 90-minute nap in the early afternoon, after being subjected to a rigorous learning task, did markedly better at later round of learning exercises, compared to those who remained awake throughout the day. The former group actually improved in their capacity to learn, while the latter became worse at learning. The findings reinforce the hypothesis that sleep is needed to clear the brain's short-term memory storage and make room for new information. Moreover, this refreshing of memory capacity was related to Stage 2 non-REM sleep (an intermediate stage between deep sleep and the REM dream stage).
The preliminary findings were presented February 21, at the annual meeting of the American Association of the Advancement of Science (AAAS) in San Diego, Calif.
Helping memory consolidation while you sleep
The role of sleep in consolidating new learning is now well-established, but now a study intriguingly reveals that you can improve that learning by playing sounds associated with the learning while you are asleep. The study involved 12 volunteers learning to associate each of 50 images with a random location on a computer screen. Each object was paired with its associated sound. Some 45 minutes after they had successfully mastered this task, each participant lay down in a quiet, darkened room. Once deeply asleep, 25 of these sounds were played. Although none of the participants noticed these sounds, performance was subsequently more accurate for those objects whose sounds had been played during sleep. The findings reveal that memory consolidation can be directed to specific memories through use of such cues. Another recent study found smells could also be used in this way.
(2009). Strengthening Individual Memories by Reactivating Them During Sleep.
Science. 326(5956), 1079 - 1079.
Sleep helps reduce errors in memory
A study in which college students were shown lists of words and then, 12 hours later, asked to identify which words they had seen or heard earlier, found that those who trained at night and tested the following morning were less prone to falsely recognizing semantically similar words than those who trained in the morning and tested in the evening. It’s suspected that sleep may help strengthen the source of the memory, thus helping protect against false memories.
(2009). Reduced false memory after sleep.
Learning & Memory. 16(9), 509 - 513.
How sleep consolidates memory
A rat study provides clear evidence that "sharp wave ripples", brainwaves that occur in the hippocampus when it is "off-line", most often during stage four sleep, are responsible for consolidating memory and transferring the learned information from the hippocampus to the neocortex, where long-term memories are stored. The study found that when these waves were eliminated during sleep, the rats were less able to remember a spatial navigation task.
(2009). Selective suppression of hippocampal ripples impairs spatial memory.
Nat Neurosci. 12(10), 1222 - 1223.
Memories practiced throughout the day, not just while sleeping
It is known that a certain amount of replaying of experiences occurs in the hippocampus immediately afterwards, but it has been thought that this is confined to the immediate past, while the replaying that occurs during sleep and is thought to be part of the memory consolidation process, ranges far more widely. Now a new rat study indicates that the replaying that occurs while the animal is awake is more extensive than thought, and more accurate than that which occurs during sleep. Data from the neurons indicated that the events being replayed (repeatedly) were from 20 to 30 minutes earlier, and involved different settings, indicating the replay wasn’t dependent on incoming sensory cues. It’s suggested that the less-accurate replays seen during sleep are more aimed at making connections, rather than consolidating the actual experience. The waking replays occurred during pauses in activity, perhaps suggesting the importance of making pauses for reflection during your day!
(2009). Awake replay of remote experiences in the hippocampus.
Nature Neuroscience. 12(7), 913 - 918.
Creative problem solving enhanced by REM sleep
A study investigating the role of sleep in creative problem-solving has found that those who experienced REM sleep between two tests performed significantly better on the later test compared to those who simply had a quiet rest, or those who napped but had no REM sleep. The findings support the idea that REM sleep (when dreams occur) has a role in forming new associations. It’s suggested that the process may be facilitated by changes to neurotransmitter systems (cholinergic and noradrenergic) during REM sleep.
(2009). REM, not incubation, improves creativity by priming associative networks.
Proceedings of the National Academy of Sciences. 106(25), 10130 - 10134.
Sleep may be important in regulating emotional responses
A study involving 44 college students who were asked to remember scenes with neutral or negative objects on a neutral background has found that those who trained and tested on the scenes in the evening remembered the negative scenes better than those who were trained and tested in the morning. However, neutral objects were not better remembered, and the backgrounds associated with negative objects were more poorly remembered by this group. The pattern persisted when the students were tested four months later. The findings suggest that the sleeping brain calculates what is most important about an experience and selects only what is adaptive for consolidation and long term storage.
Payne, J.D., Kensinger, E., Wamsley, E. & Stickgold, R. 2009. Sleep Promotes Lasting Changes in Memory for Emotional Scenes. Presented on June 11 at SLEEP 2009, the 23rd Annual Meeting of the Associated Professional Sleep Societies; Abstract ID: 1244.
Sleep may help clear the brain for new learning
Although fruit flies may seem little like us, their response to sleep deprivation is similar, and so they are useful models for sleep effects on the human brain. In a recent study, flies genetically altered to make it easier to track individual synapses have revealed that during sleep the number of new synapses formed during earlier learning decreased. This decline didn’t happen if the flies were deprived of sleep. It’s theorized that this activity during sleep is a way of pruning the less relevant and important synapses (clearing away the junk, as it has been conceptualized). The study follows earlier fruit fly research showing that more learning resulted in longer sleep. It also supports recent rat research that found synaptic strength increases during the day, then weakens during sleep. The study also identified three genes essential to the links between learning and increased need for sleep, one of which is equivalent to a human gene known as serum response factor (SRF) and previously linked to brain plasticity.
(2009). Use-Dependent Plasticity in Clock Neurons Regulates Sleep Need in Drosophila.
Science. 324(5923), 105 - 108.
Sleep helps you learn complicated tasks & recover forgotten skills
A study involving 200 mostly female college students, who had little experience of video games. The students were taught to play a complicated, multisensory video game in which players must use both hands to deal with continually changing visual and auditory signals. Half were tested 12 hours after the training session, and the others 24 hours later. Some were given a night’s sleep before testing, others were tested the same day. Performance in the former dropped by half at testing, but when tested again the following morning, they showed a 10 percentage point improvement over their pre-test performance. For those given evening training, scores improved by about 7 percentage points, then went to 10 percentage points the next morning – which was maintained over the day. The findings indicate that although people may appear to forget much of their learning over the course of a day, a night’s sleep will restore it; moreover, sleep protected the memory from loss over the course of the next day. The findings confirm the role of sleep in consolidating memory for skills, and extends the research to complicated tasks.
(2008). Consolidation of sensorimotor learning during sleep.
Learning & Memory. 15(11), 815 - 819.
Sleep selectively preserves emotional memories
It’s now generally accepted that sleep plays an important role in consolidating procedural (skill) memories, but the position regarding other types of memory has been less clear. A new study has found that sleep had an effect on emotional aspects of a memory. The study involved showing 88 students neutral scenes (such as a car parked on a street in front of shops) or negative scenes (a badly crashed car parked on a similar street). They were then tested for their memories of both the central objects in the pictures and the backgrounds in the scenes, either after 12 daytime hours, or 12 night-time hours, or 30 minutes after viewing the images, in either the morning or evening. Those tested after 12 daytime hours largely forgot the entire negative scene, forgetting both the central objects and the backgrounds equally. But those tested after a night’s sleep remembered the emotional item (e.g., the smashed car) as well as those who were tested only 30 minutes later. Their memory of the neutral background was however, as bad as the daytime group. The findings are consistent with the view that the individual components of emotional memory become 'unbound' during sleep, enabling the brain to selectively preserve only that information it considers important.
(2008). Sleep preferentially enhances memory for emotional components of scenes.
Psychological Science: A Journal of the American Psychological Society / APS. 19(8), 781 - 788.
Aging impairs the 'replay' of memories during sleep
During sleep, the hippocampus repeatedly "replays" brain activity from recent experiences, in a process believed to be important for memory consolidation. A new rat study has found reduced replay activity during sleep in old compared to young rats, and rats with the least replay activity performed the worst in tests of spatial memory. The best old rats were also the ones that showed the best sleep replay. Indeed, the animals who more faithfully replayed the sequence of neural activity recorded during their earlier learning experience were the ones who performed better on the spatial memory task, regardless of age. The replay activity occurs during slow-wave sleep.
(2008). Sequence Reactivation in the Hippocampus Is Impaired in Aged Rats.
J. Neurosci.. 28(31), 7883 - 7890.
A nap can help you learn
A study of 33 younger adults (average are 23) has found that a 45 minute afternoon nap (containing only non-REM sleep) improved performance on 3 different declarative memory tasks, but only when the subjects had reached a certain level of performance during training.
(2008). Enhancement of declarative memory performance following a daytime nap is contingent on strength of initial task acquisition.
Sleep. 31(2), 197 - 203.
Brain connections strengthen during waking hours, weaken during sleep
New research provides support for a much-debated theory that we need sleep to give our synapses time to rest and recover. The human brain is said to expend up to 80% of its energy on synaptic activity, constantly adding and strengthening connections in response to stimulation. The researchers have theorized that we need an ‘off-line period’, when we are not exposed to the environment, to take synapses down. The rodent study has revealed by several measures that synapses — the all-important points of connection between neurons — are very active when the animal is awake and very quiet during sleep. The researchers feel that these findings support the idea that our brain circuits get progressively stronger during wakefulness and that sleep helps to recalibrate them to a sustainable baseline. This theory is of course opposite to the currently dominant hypothesis, that during sleep synapses are hard at work replaying the information acquired during the previous waking hours, consolidating that information by becoming even stronger.
(2008). Molecular and electrophysiological evidence for net synaptic potentiation in wake and depression in sleep.
Nat Neurosci. 11(2), 200 - 208.
Sleep reinforces the temporal sequence in memory
Following on from research showing long-term memory is consolidated during sleep through the replaying of recently encoded experiences, a study has found that the particular order in which they were experienced is also strengthened, probably by a replay of the experiences in "forward" direction. The study involved students being asked to learn triplets of words presented one after the other. Those whose recall of the order of the words was tested after sleep showed better recall, but only when they were asked to reproduce the learned words in forward direction.
(2007). Sleep Enforces the Temporal Order in Memory.
PLoS ONE. 2(4), e376 - e376.
Sleep protects against interference
A study involving 48 people (aged 18—30) found that those who learned 20 pairs of words at 9pm and were tested at 9am the following morning, after a night’s sleep, performed better than those who learned them at 9am and were tested at 9pm of the same day. Moreover, for those who were given a second list of word pairs to remember just before testing, where the first word in each pair was the same as on the earlier list, the advantage of sleep was dramatically better. For those who experienced the interference manipulation, those in the sleep group recalled 12% more word pairs than the wake group, but with interference, the recall rate was 44% higher for the sleep group.
The findings were presented by Dr Jeffrey Ellenbogen at the American Academy of Neurology’s 59th Annual Meeting in Boston, April 28 – May 5, 2007.
Sleeping helps us put facts together
And in yet another sleep study, researchers found evidence that sleep also helps us see the big picture. The study involved 56 students who were shown oval images of colorful abstract patterns nicknamed "Fabergé eggs." Participants were first shown a combination of five pairs of the eggs, all of which were given ratings. The students were given 30 minutes to learn which shape rated higher and so should be chosen over another shape. They were not told the hidden connection that linked all five pairs together. They were then tested either after 20 minutes, after 12 hours, or after 24 hours. Half of those in the 12-hour group slept before the test, the other half did not. The 20-minute group performed the worst, showing no evidence of seeing the pattern. Those who had longer before being tested were much more likely to show signs of inferential judgment (75% vs 52%), and for the most distant (and difficult) inferential judgment, the students who had had periods of sleep in between learning and testing significantly outperformed those who hadn’t slept (93% vs 69%). The researchers are interested in exploring whether meditation can provide a similar benefit.
(2007). Human relational memory requires time and sleep.
Proceedings of the National Academy of Sciences. 104(18), 7723 - 7728.
More on how memories are consolidated during sleep
A new study sheds more light on how memory is consolidated during sleep. Using a new technique, the research confirms that new information is transferred between the hippocampus and the cerebral cortex, and, unexpectedly, provides evidence suggesting that the cerebral cortex actively controls this transfer.
(2006). Phase-locking of hippocampal interneurons' membrane potential to neocortical up-down states.
Nat Neurosci. 9(11), 1359 - 1361.
Still more on how memories are consolidated during sleep
In research following up an earlier study in which rats were shown to form complex memories for sequences of events experienced while they were awake, and that these memories were replayed while they slept, it has been shown that these replayed memories do contain the visual images that were present during the running experience. By showing that the brain is replaying memory events in the visual cortex and in the hippocampus at the same time, the finding suggests that this process may contribute to or reflect the result of the memory consolidation process.
(2007). Coordinated memory replay in the visual cortex and hippocampus during sleep.
Nat Neurosci. 10(1), 100 - 107.
Brainwave oscillations responsible for memory benefits of sleep?
Passing a mild electrical current through the brain while students were asleep improved their ability to remember words on waking up. 13 medical students were given 46 pairs of words to learn. Before sleeping, they remembered an average 37.42 words; after sleep, those not given the stimulation remembered an average of 39.5, while those given the stimulation remembered an average of 41.27. The memory enhancement only occurred at a certain frequency and during a particular part of the sleep cycle, confirming the idea that slow oscillations of electrical activity are responsible for the memory consolidation effects of sleep. The benefit also only applied to fact learning; skill learning was not affected.
(2006). Boosting slow oscillations during sleep potentiates memory.
Nature. 444(7119), 610 - 613.
More support that sleep helps consolidate learning
An experiment involving fruitflies has found that those in a social environment with at least 30 other flies slept four times as long during their daytime naps as flies in isolation. There was no difference in night-time sleep. The length of the nap increased with the size of the group they socialized with. Confirming that this effect was due to an increase in social interactions, rather than, for example, physical exhaustion from flying around more, flies deprived of their sight and sense of smell (meaning they could still fly around but could not socialize) showed no difference in daytime sleep patterns. Of 49 genes known to be involved in learning and memory, switching off seventeen (all related to long-term memory) made the flies sleep equally long regardless of whether they were social or not.
(2006). Waking Experience Affects Sleep Need in Drosophila.
Science. 313(5794), 1775 - 1781.
Human study supports value of daytime napping for learning
REM sleep, when most dreaming occurs, has been shown in a number of studies to be important in consolidating procedural (skill) learning, while non-REM (slow-wave) sleep seems to be more important for declarative (knowledge-based) learning. However, because normal sleep contains both REM and non-REM cycles, research hasn’t been able to clearly distinguish the effects. Now a new study using brief daytime napping confirms the role of non-REM sleep for declarative learning. Volunteers who memorized pairs of words and practiced tracing images in a mirror test scored 15% better in the word test if they had been allowed a nap in the six hour period before being tested. However, they did no better at the action test.
(2006). A daytime nap containing solely non-REM sleep enhances declarative but not procedural memory.
Neurobiology of Learning and Memory. 86(2), 241 - 247.
Sleep makes memories resistant to interference
It’s pretty clear now that sleep consolidates procedural (skill) learning, but the question of whether or not it helps other types of memory is still very much a matter of debate. However, a new study has found a marked effect of sleep on our ability to remember information. The study involved 60 healthy college-aged adults, who were asked them to memorize 20 pairs of random words. Half were given the words at 9am and tested at 9pm, and the other half were given the words at 9pm and tested at 9am. While the sleepers did perform better (94% recall compared to 82%), it was the introduction of another factor that made the benefits of sleep undeniable. Participants who were given a new set of words to learn just 12 minutes before testing revealed a dramatic difference — sleepers recalled 76% of the original words compared to 32% of the sleepless.
(2006). Interfering with Theories of Sleep and Memory: Sleep, Declarative Memory, and Associative Interference.
Current Biology. 16(13), 1290 - 1294.
Asleep or awake we retain memory
We’ve learned that skill memory is reinforced during sleep, but now new imaging technology reveals that this kind of reinforcement occurs while we’re awake too — even while we’re learning something new.
(2006). Offline Persistence of Memory-Related Cerebral Activity during Active Wakefulness.
PLoS Biol. 4(4), e100 - e100.
How sleep improves memory
While previous research has been conflicting, it does now seem clear that sleep consolidates learning of motor skills in particular. A new imaging study involving 12 young adults taught a sequence of skilled finger movements has found a dramatic shift in activity pattern when doing the task in those who were allowed to sleep during the 12 hour period before testing. Increased activity was found in the right primary motor cortex, medial prefrontal lobe, hippocampus and left cerebellum — this is assumed to support faster and more accurate motor output. Decreased activity was found in the parietal cortices, the left insular cortex, temporal pole and fronto-polar region — these are assumed to reflect less anxiety and a reduced need for conscious spatial monitoring. It’s suggested that this is one reason why infants need so much sleep — motor skill learning is a high priority at this age. The findings may also have implications for stroke patients and others who have suffered brain injuries.
(2005). Sleep-dependent motor memory plasticity in the human brain.
Neuroscience. 133(4), 911 - 917.
More evidence that learning is consolidated during sleep
A new study provides more evidence for the role of sleep in the consolidation of long-term memories. In the study, volunteers learned the layout of a virtual town, and were then tested by having to quickly find routes to various locations in the town. Those so trained showed greater activity in their hippocampus and an adjacent learning-related region (compared to those not trained) as they took the route tests, with greater activity correlated with better performance. They also showed greater hippocampal brain activity during sleep. Most importantly, the higher the gain in post-sleep performance on the tests, the higher had been their NREM brain activity during sleep. No such correlation was found in REM brain activity. The findings support the view that spatial memory traces are processed during NREM sleep in humans.
(2004). Are spatial memories strengthened in the human hippocampus during slow wave sleep?.
Neuron. 44(3), 535 - 545.
Mentally, sleep may be as active a state as waking state
Why do we sleep? A question we keep asking. Recent research leads us another step in the road. The study has identified a number of genes upregulated specifically during sleep – at least as many as are turned on while we are awake. These "sleep genes" largely fall into four categories: genes involved in synaptic plasticity (supporting the view that sleep aids memory consolidation); genes underlying translation (supporting observations that protein synthesis increases during sleep); genes regulating membrane and vesicle trafficking; and genes for synthesizing cholesterol (which may be crucial for synapse formation and maintenance, which could, in turn, enhance neural plasticity (the brain's ability to change and learn)). The study also found, to the researchers’ surprise, that the cerebellum showed largely the same pattern of gene-expression during sleep as the cortex.
(2004). Extensive and divergent effects of sleep and wakefulness on brain gene expression.
Neuron. 41(1), 35 - 43.
More on what goes on during sleep
Brain activity patterns vary during sleep, with particular distinction being made between “REM” sleep and “deep” sleep. Both these phases of sleep have been associated with memory processing. The chemical composition of the brain also varies a great deal in the sleep and wakefulness cycle. New research from Germany now report that some of these differences are crucial in memory formation during sleep. In particular, the level of acetylcholine (a neurotransmitter) is high during wakefulness and REM sleep but drops to the minimum in deep sleep. In an experiment that involved subjects performing two memory tasks – learning 40 pairs of semantically related words, and learning to trace figures seen in a mirror – before sleeping for four hours, it was found that those who were given a cholinesterase inhibitor, (cholinesterase being an enzyme that breaks down acetylcholine), performed significantly less well in the wordlist task on wakening. The mirror-tracing task didn't seem to be affected. This supports the idea that a low level of acetylcholine is necessary for strengthening explicit memory during deep sleep, and fits in with a proposed two-stage model of long-term memory formation, in which the cortex transfers newly acquired experiential data to the hippocampus for processing and temporary storage (a process requiring high levels of acetylcholine), and then, during sleep, the processed memory traces in the hippocampus are relayed back to the cortex for long-term storage. This feedback process is blocked by acetylcholine and, thus, only happens in sleep when the acetylcholine level drops to the minimum.
The research may also have important implications for treating memory loss associated with Alzheimer's disease, as cholinesterase inhibitors are widely used in such treatment. Because of common side-effects of the drug, patients are usually told to take it at night, which may well weaken the drug’s effectiveness.
(2004). Low acetylcholine during slow-wave sleep is critical for declarative memory consolidation.
Proceedings of the National Academy of Sciences of the United States of America. 101(7), 2140 - 2144.
Now definite? Memories are consolidated during sleep
Researchers of a new study claim that their research finally settles the question of whether or not sleep consolidates new memories. The study involved detailed recording of specific learning- and memory- related areas (hippocampus and forebrain) in the brains of rats. The rats were exposed to four kinds of novel objects. Analysis of brain signals before, during, and after this experience, revealed "reverberations" of distinctive brain wave patterns across all the areas being monitored for up to 48 hours after the novel experience. This pattern was much more prevalent in slow-wave sleep than in REM sleep. Previous studies by the same researchers have found that the activation of genes that affect memory consolidation occurs during REM sleep, not slow-wave sleep. It is proposed that both stages of sleep are important for memory consolidation. Previous studies have tended to focus solely on the hippocampus, and have observed brain activity for a much shorter period.
(2004). Long-lasting novelty-induced neuronal reverberation during slow-wave sleep in multiple forebrain areas.
PLoS Biology. 2(1), E24 - E24.
Full text available at http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0020037
Sleep helps insight
A new German study provides evidence for what we all suspected — “sleeping on” a problem can really work. In the study, participants were given a mathematical puzzle to solve; a puzzle which could be solved by trial-by-trial learning, or almost immediately if participants grasped the hidden rule. After training in the trial-by-trial learning, some of the participants were allowed to sleep through the night, while others were prevented from sleeping. When they returned to the problem eight hours later, those that had slept were twice as likely to realize the rule. Another group that trained in the morning, and were then tested later that day, were also slower at finding the rule, suggesting that the slowness was not solely due to fatigue. Sleep did not, however, help participants who had not had the initial training. It is suggested that sleep can act to restructure new memory representations.
(2004). Sleep inspires insight.
Nature. 427(6972), 352 - 355.
Stages of memory clarified in sleep studies
Two new studies add to our understanding of the effects of sleep on memory. Both studies involved young adults and procedural (skill) learning, and found temporary declines in performance in particular contexts (a brief description of these studies is given here). On the basis of these studies, researchers identified three stages of memory processing: the first stage of memory — its stabilization — seems to take around six hours. During this period, the memory appears particularly vulnerable to being “lost”. The second stage of memory processing — consolidation — occurs during sleep. The third and final stage is the recall phase, when the memory is once again ready to be accessed and re-edited. (see my article on consolidation for more explanation of the processes of consolidation and re-consolidation). The surprising aspect to this is the time it appears to take for memories to initially stabilize. The studies also confirm the role of sleep in the consolidation process.
(2003). Consolidation during sleep of perceptual learning of spoken language.
Nature. 425(6958), 614 - 616.
(2003). Dissociable stages of human memory consolidation and reconsolidation.
Nature. 425(6958), 616 - 620.
More support for the theory that sleep is necessary to consolidate memories
A study used fear conditioning in mice to investigate the effect of sleep deprivation on memory. The mice were given a mild electric shock either in a distinctive setting, or subsequent to a tone. Those who experienced the tone continued to freeze when they heard the tone on the following day, whether or not they had been deprived of sleep. Those who associated the environment with the shock, however, were less likely to freeze after sleep deprivation. Mice who had been deprived of sleep during the five hours following training, spent just 4% of their time frozen when returned to the ‘shock environment’ the following day, compared to 15% among mice who were allowed to sleep during this period. The five hours following training was a critical period – those who were deprived of sleep in the 5-10 hours after training showed no sign of memory impairment. The fact that the context association was affected but not the tone cue, suggests that sleep is affecting processes in the hippocampus (important in context memory but not memory for specific facts or events).
(2003). Sleep Deprivation Selectively Impairs Memory Consolidation for Contextual Fear Conditioning.
Learning & Memory. 10(3), 168 - 176.
Another step in understanding how sleep affects memory
The value of sleep for memory takes a further step in being understood in new rodent research, which found that, as the rodents slept, the thalamus at the base of their brains originated bursts of electrical activity (“sleep spindles”), which were then detected in the somatosensory neocortex. Some 50 msec later, the hippocampus responded with a pulse of electricity (a “ripple”). "This neocortical-hippocampal dialogue may provide a selection mechanism for the time-compressed replay of information learned during the day." It’s suggested that the ripple is the hippocampus sending back neat, compact waves of memory to the neocortex where they are filed away for future reference. Most of this activity took place during slow wave sleep, the stage which makes up the majority of the sleep cycle.
(2003). Single Neurons in the Monkey Hippocampus and Learning of New Associations.
Science. 300(5625), 1578 - 1581.
Napping reverses information overload
Evidence is mounting that sleep helps information processing and learning. A new study has showed that subjects performing a visual task (reporting the horizontal or vertical orientation of three diagonal bars against a background of horizontal bars in the corner of a computer screen) got worse over the course of four daily practice sessions. However, allowing subjects a 30-minute nap after the second session prevented any further deterioration, and a 1-hour nap actually boosted performance in the third and fourth sessions back to morning levels. It appears that the fatigue is limited to the brain visual system circuits involved in the task. When the image was switched to a different right corner of the computer screen on the fourth practice session, subjects performed about as well as they did in the first session -- or after a short nap. Recordings of brain activity reveal that the 1-hour naps contained more than four times as much deep, or slow wave sleep and rapid eye movement (REM) sleep than the half-hour naps.
(2002). The restorative effect of naps on perceptual deterioration.
Nat Neurosci. 5(7), 677 - 681.
Improving motor skills through sleep
People taught a simple motor sequence (to type a sequence of keys on a computer keyboard as quickly and accurately as possible) practised it for 12 minutes and were then re-tested 12 hours later. Those who practised in the morning and tested later that same day improved their performance by about 2%. Those trained in the evening and re-tested after a good night's sleep, however, improved by about 20%. The amount of improvement was directly correlated with the amount of Stage 2 (a stage of non-rapid eye movement or NREM) sleep experienced, particularly late in the night. "This is the part of a good night's sleep that many people will cut short by getting up early in the morning."
(2002). Sleep and motor skill learning.
Neuron. 35(1), 5 - 7.
Controversy over sleep's role in memory
Does sleep play a role in memory or not? Two new research papers reach opposite conclusions. One is from Robert Stickgold, who has published several papers supporting the role of sleep in memory consolidation. But the other is a new review of REM sleep studies concluding that REM (rapid eye movement) sleep, or dreaming, plays little role in memory formation, chiefly on the basis that depriving animals and humans of REM sleep by awakening them or by drug treatments does not impair their ability to form long-term memories. In addition, the time spent in REM sleep does not correlate with learning ability across humans, nor is there a positive relation between amount or intensity of REM sleep and learning ability across species.
(2001). Sleep, Learning, and Dreams: Off-line Memory Reprocessing.
Science. 294(5544), 1052 - 1057.
(2001). The REM sleep-memory consolidation hypothesis.
Science (New York, N.Y.). 294(5544), 1058 - 1063.
New motor skills consolidated during sleep
An imaging study that sheds light on the gain in performance observed during the day after learning a new task. Following training in a motor skill, certain brain areas appear to be reactived during REM sleep, resulting in an optimization of the network that subtends the subject's visuo–motor response.
(2001). Experience-dependent changes in cerebral functional connectivity during human rapid eye movement sleep.
Neuroscience. 105(3), 521 - 525.
Deep "slow wave" sleep necessary to consolidate memories
Sleep is necessary to consolidate memories. Remembering a new task is more difficult if you don't sleep within 30 hours of learning the task. "Catch-up" sleep on subsequent nights doesn't make up for losing that first night's sleep. Moreover, it appears that the deep "slow wave" sleep that occurs in the first half of the night is the type of sleep necessary to consolidate memories. Other types of memory however, may require "REM" sleep (that occurs while you are dreaming).
Stickgold, R., James, L. & Hobson, J.A. 2000. Visual discrimination learning requires sleep after training. Nature Neuroscience,3, 1237-1238.