A Night's Sleep Gives Emotional Memories
For the first time, researchers have found that following a night's sleep, emotional components of scenes are remembered at the expense of neutral components. In contrast, memories of both emotional and neutral components decayed equally following 12 hours of wakefulness. Sleep also promoted memory for generality over detail, says NIMH grantee Robert Stickgold, Ph.D., of Harvard University, who co-authored the recent study of memory consolidation.
Yet sleep enhanced memory of emotionally-charged objects at a cost. Things in the background of emotional scenes were forgotten more than in scenes containing neutral objects - what the researchers call the "emotional memory trade-off."
"During sleep, individual components of a scene seem to become unbound," explained Stickgold. "This allows us to selectively preserve only what's most important and worth remembering. Physiological and chemical changes in the brain during sleep appear to actively give emotional memories their staying power."
The new study is the first to demonstrate the emotional memory trade-off over time, and to show how individual components of emotional scene memories evolve differently across time spent asleep and awake. Of a few studies to date that have addressed these issues, none have detected benefit from sleep for neutral memories, say the researchers.
Harvard's Jessica Payne, Ph.D., Boston College's Elizabeth Kensinger, Ph.D., Stickgold, and colleagues, report on their findings in the August, 2008 issue of the journal Psychological Science.
Problems with sleep and emotional memories figure prominently in mood and anxiety disorders, and the consequences of inadequate sleep loom for the third of Americans whose sleep-deprived lifestyles interfere with work and social functioning at least a few days each month. The new study is the latest in a series by the Harvard group on how sleep helps consolidate memory.
Based on hints from previous studies, the researchers set out to determine exactly which aspects of memories of emotional events are influenced by sleep. They presented a total of 88 participants with mixed-and-matched sets of pictures of neutral or negatively arousing objects superimposed on neutral or negative backgrounds.
Separate memory tests 12 hours later showed that memory for both objects and backgrounds deteriorated equally, regardless of emotional content, in subjects who studied the pictures in the morning, stayed awake all day, and were tested the same evening. But the disparity in recognition for negative emotional objects vs. their backgrounds more than doubled for participants who were trained in the evening, slept overnight, and were tested the following morning. This increase was entirely due to maintenance of emotional-object memory during sleep, say the researchers. Both after 30 minutes and at 12 hours, memory of negative objects' backgrounds suffered relative to that for neutral objects.
"There is a popular belief that the consequence of sleep deprivation is simply tiredness, and that a good night's sleep on the weekend can completely reverse any deleterious effects of mid-week deprivation," notes Stickgold.
"The many college students, medical, public safety, and transportation personnel who practice such 'sleep bulimia' depend on continued learning for the effective performance of their tasks. A clear understanding of exactly how sleep loss contributes to a failure of memory consolidation should provide important arguments to help counter this cultural drift towards less and less sleep," Stickgold adds.