The brain cherry-picks what people remember during sleep, resulting in sharper and clearer thinking, a new study suggests.
Previous research had shown that sleep helps people consolidate their memories, fixing them in the brain so we can retrieve them later. (Read about secrets of why we sleep in National Geographic magazine.)
But the new study, a review based on new studies as well as past research on sleep and memory, suggests that sleep also transforms memories in ways that make them somewhat less accurate but more useful in the long run.
For example, sleep-enabled memories may help people produce insights, draw inferences, and foster abstract thought during waking hours.
"The sleeping brain isn't stupid—it doesn't just consolidate everything you put into it, but calculates what to remember and what to forget," said study leader Jessica Payne, a cognitive neuroscientist at the University of Notre Dame in Indiana.
(Also see: "Naps Clear Brain's Inbox, Improve Learning.")
Emotional Memories Stick
For instance, the memory details that seem to get remembered best are often the most emotional ones, Payne said.
Payne and colleagues found that when people are shown a scene with an emotion-laden object in the foreground—such as a wrecked car—they are more likely to remember that object than, say, palm trees in the background, especially if they are tested after a night of slumber.
Rather than preserving scenes in their entirety, the brain apparently restructures scenes to remember only their most emotional and perhaps most important elements while allowing less emotional details to deteriorate.
Measurements of brain activity support this notion, revealing that brain regions linked with emotion and memory consolidation are periodically more active during sleep then when awake.
"It makes sense to selectively remember emotional information—our ancestors would not want to forget a snake was in a particular location or that someone in the tribe was particularly mean and should be avoided," said Payne, whose study appeared in the October issue of the journal Current Directions in Psychological Science.
"Memories are not so much about remembering the past as being able to anticipate and predict multiple possible futures."
Selective Memory's Dark Side
But there are dark sides to such selectivity. For instance, the brain can focus on remembering negative experiences at the exclusion of others, which occurs in depression and post-traumatic stress disorder. (Related: "Beyond the Brain" in National Geographic magazine.)
Future research may shed light on what details are remembered and how they're remembered, which could help deal with trauma, Payne noted.
"You could also see such work being helpful in coming up with solutions in the classroom or in the business world," she said.
Future research may also reveal what components of sleep might be linked with these mental processes.
"Does it require the REM sleep associated with dreaming, or deeper slow-wave sleep?" said Robert Stickgold, a cognitive neuroscientist at Harvard Medical School who researches sleep.
Overall, "sleep is doing much more complicated stuff than just stabilizing or strengthening memories," Stickgold added.
"We're seeing the sorts of memory processing in sleep that we usually attribute to cleverness."