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.
 . Processing of Emotional Reactivity and Emotional Memory over Sleep. The Journal of Neuroscience [Internet]. 2012 ;32(3):1035 - 1042. Available from: http://www.jneurosci.org/content/32/3/1035.abstract