The role of sleep in memory: Research reports

What sleep does for memory

April 2009

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 theorised that this activity during sleep is a way of pruning the less relevant and important synapses (clearing away the junk, as it has been conceptualised). 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.
The study was published in the April 3 edition of Science.  Full reference
http://www.eurekalert.org/pub_releases/2009-04/wuso-smh033109.php
http://www.eurekalert.org/pub_releases/2009-04/uow-ssc033009.php

November 2008

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.
The report appeared in the November issue of Learning and Memory.  Full reference
http://www.eurekalert.org/pub_releases/2008-11/uoc-shp111708.php

August 2008

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.
The study was reported in the August issue of Psychological Science.  Full reference
http://www.physorg.com/news137908693.html
http://www.eurekalert.org/pub_releases/2008-08/bidm-sft081308.php

July 2008

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.
The research was reported in the July 30 issue of The Journal of Neuroscience.  Full reference
http://www.eurekalert.org/pub_releases/2008-07/sfn-ait072408.php

February 2008

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.
The study was published in the February 1 issue of SLEEP. Full reference
http://www.eurekalert.org/pub_releases/2008-02/aaos-jss012808.php

January 2008

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.
The report appeared online January 20 in Nature Neuroscience. Full reference
http://www.physorg.com/news120059987.html

April 2007

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.
The paper appeared in the April 18 issue of PLoS ONE. Full reference
http://www.eurekalert.org/pub_releases/2007-04/plos-set041707.htm

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.
http://www.eurekalert.org/pub_releases/2007-04/aaon-ssy040307.htm

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.
The findings appeared online April 20 in the Early Edition of the Proceedings of the National Academy of Sciences. Full reference
http://www.physorg.com/news98376198.html
http://www.eurekalert.org/pub_releases/2007-04/bidm-tut042007.htm

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.
The study appeared in the November issue of Nature Neuroscience. Full reference
http://www.eurekalert.org/pub_releases/2006-12/m-lds120506.htm

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.
The report appeared December 17 in the advance online edition of Nature Neuroscience. Full reference
http://www.eurekalert.org/pub_releases/2006-12/miot-mtr121806.htm

October 2007

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.
The results were published online 5 November in Nature. Full reference
http://www.guardian.co.uk/science/story/0,,1940475,00.html
http://www.sciam.com/article.cfm?chanID=sa003&articleID=BEC346B2-E7F2-99DF-350CC33BA6757700
http://www.nature.com/news/2006/061030/full/444133a.html

September 2006

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.
The study was reported in the September 22 issue of Science. Full reference
http://www.nature.com/news/2006/060918/full/060918-9.html
http://www.livescience.com/humanbiology/060921_flies_sleep.html

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.
The report appeared in the September issue of Neurobiology of Learning and Memory. Full reference
http://www.newscientist.com/article/mg19125704.800?DCMP=NLC-nletter&nsref=mg19125704.800

July 2006

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.
The findings are reported in the July 12 issue of Current Biology. Full reference
http://www.sciencedaily.com/releases/2006/07/060711095912.htm
http://www.sciam.com/article.cfm?chanID=sa003&articleID=0006A257-BBB4-14B2-B8B983414B7F4945

March 2006

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.
The study was published in PLoS Biology. Full reference
http://www.eurekalert.org/pub_releases/2006-03/plos-aoa032206.htm
http://www.sciencedaily.com/releases/2006/03/060329085308.htm

June 2005

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.
The findings were reported in the June 30 issue of Neuroscience. Full reference http://www.eurekalert.org/pub_releases/2005-06/bidm-ssh062805.htm

October 2004

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.
The study appeared in the October 28 issue of Neuron. Full reference
http://www.eurekalert.org/pub_releases/2004-10/cp-etl102204.htm

February 2004

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.
The study was published in the January 8 issue of Neuron. Full reference
http://www.the-scientist.com/yr2004/feb/research2_040216.html

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.
The study was published in the February 17 issue of Proceedings of National Academy of Sciences. Full reference
http://gateways.bmn.com/neuroscience/news?uid=NEWS.040219-1

January 2004

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.
The researchers published their findings on Jan. 19, 2004, in the online Public Library of Science. Full reference
http://www.eurekalert.org/pub_releases/2004-01/dumc-etm011304.htm
http://www.eurekalert.org/pub_releases/2004-01/plos-brd011204.htm
http://www.plosbiology.org/plosonline/?request=get-document&doi=10.1371/journal.pbio.0020037
Full text: http://www.plosbiology.org/plosonline/?request=get-document&doi=10.1371%2Fjournal.pbio.0020024

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.
The study was published on 22 January in Nature. Full reference
http://www.sciam.com/article.cfm?chanID=sa003&articleID=000088CE-E9DC-100E-A9DC83414B7F0000
http://www.sfgate.com/cgi-bin/article.cgi?file=/news/archive/2004/01/21/national0259EST0431.DTL
http://www.nature.com/nsu/040119/040119-10.html
http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v427/n6972/abs/nature02223_fs.html

October 2003

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.
The studies appeared in the October 9 issue of Nature. Full reference 2
http://www.eurekalert.org/pub_releases/2003-10/bidm-som100703.htm
http://www.sciencenews.org/20031011/fob4.asp
http://education.guardian.co.uk/higher/research/story/0,9865,1059138,00.html

July 2003

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).
The results were reported in the May/June issue of Learning & Memory. Full reference
http://www.eurekalert.org/pub_releases/2003-07/uop-sdw070803.htm

June 2003

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.
The findings are reported in the June 6 issue of Science. Full reference
http://www.eurekalert.org/pub_releases/2003-06/nyu-fir060503.htm
http://tinyurl.com/ftob

July 2002

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.
The study was reported in the July 1 issue of Nature Neuroscience. Full reference
http://www.eurekalert.org/pub_releases/2002-07/niom-np070102.htm

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."
The study appeared in the July 3 issue of Neuron. Full reference
http://www.eurekalert.org/pub_releases/2002-07/hms-pmp070102.htm

November 2001

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.
The articles appear in the November 2 edition of Science. Full references
1, 2
http://www.sciencemag.org/cgi/content/abstract/294/5544/1052
http://www.sciencemag.org/cgi/content/abstract/294/5544/1058

October 2001

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.
The report appeared in the October issue of Neuroscience. Full reference

November 2000

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).
The study was published in the December issue of Nature Neuroscience.
http://www.independent.co.uk/story.jsp?story=6296

Stickgold, R., James, L. & Hobson, J.A. 2000. Visual discrimination learning requires sleep after training. Nature Neuroscience,3, 1237-1238.