Lead author Dr Jessica Payne, of Harvard Medical School in Boston MA has revealed that sleeping brain seems to calculate what is most important about an experience and selects only what is adaptive for consolidation and long term storage.
She said that it was surprising that in addition to seeing the enhancement of negative memories over neutral scenes, there was also selectivity within the emotional scenes themselves, with sleep only consolidating what is most relevant, adaptive and useful about the scenes.
"It may be that the chemical and physiological aspects of sleep underlying memory consolidation are more effective if a particular memory is 'tagged' shortly prior to sleeping," said Payne.
During the study, the researchers looked at 44 college students between the ages of 18 and 22 who encoded scenes with neutral or negative objects on a neutral background and were tested on memory for objects and backgrounds 24 hours later.
The participants were randomly assigned to 'sleep first' group, which trained and tested on the scenes between the hours of 7 and 9 p.m. while the other half was assigned to the 'wake-first' group which trained and tested on the scenes between the hours of 9 and 11 a.m.
The study showed that negative, but not neutral objects were better remembered in the sleep-first than wake-first group.
And the backgrounds associated with negative, but not neutral objects were more poorly remembered in the sleep-first compared to the wake-first group.
Thus, while negative object memory was enhanced in the sleep-first group compared to the wake-first group, memory for the backgrounds on which they were presented was impaired in the sleep-first group compared to the wake-first group.
This pattern persisted four months later, with emotional objects being preferentially retained in the sleep-first group only.
Payne said that sleep is beneficial for memory and that we remember things best when we 'stagger' our learning episodes across time.
The study was presented at SLEEP 2009, the 23rd Annual Meeting of the Associated Professional Sleep Societies. (ANI)