Sleep on It: Time delay plus slumber equals memory boost

Sleep revs up a person’s ability to discern connections among pieces of information encountered in novel situations, a new investigation finds.

So-called relational memory, a key to flexible decision making, improves as time passes after exposure to new information. It gets an extra boost from sleep, say neuroscientist Jeffrey M. Ellenbogen of Harvard Medical School in Boston and his colleagues.

Volunteers learned to pick certain items from each of five pairs of abstract patterns on a computer screen. Study participants who then waited 12 hours—but not others who waited only 20 minutes—could identify basic relationships among the items, regardless of whether they had slept during the delay. However, only participants who waited 12 or 24 hours and slept during that time showed insight into complex relationships among the pattern pairs, the researchers report in an upcoming Proceedings of the National Academy of Sciences.

“The sleeping brain searches for distant relationships in remembered information as a way to develop general rules for dealing with new situations,” says Harvard’s Matthew P. Walker, a coauthor of the new study.

The scientists trained 56 healthy young adults to choose among six colored patterns as they appeared in a series of five pairings. In the training sessions, experimenters presented in random order the pairs that defined the range of most-preferred to least-preferred patterns. Participants were told that they had made the correct choice when they selected pattern A over pattern B, B over C, C over D, D over E, and E over F.

After varying time delays, volunteers were tested for their knowledge of the hierarchy of patterns. For instance, discerning short-range relationships included choosing pattern B over pattern D, although participants had not previously seen such pairings. Solving long-range relationships included selecting pattern B over pattern E or F.

After a 20-minute, a 12-hour, or a 24-hour delay, volunteers displayed comparably good memories for what they had initially learned, such as picking pattern A over pattern B. However, the 12 individuals tested after 20 minutes correctly discerned the more-preferred item in new pairings, such as B over D, only about half the time, indicating that they were guessing.

In contrast, 31 people were given a 12-hour delay. Of these, 14 were trained in the evening and tested after a night’s sleep. The rest were trained in the morning and tested later the same day. Thirteen others were tested after 24 hours that included a night’s sleep. These three groups all correctly detected 75 percent of short-range relationships.

The 27 participants who had slept did even better at identifying long-range relationships, correctly spotting them 93 percent of the time. After a 12-hour delay without sleep, the corresponding detection rate reached only 69 percent.

Participants who had slept weren’t aware of their newfound knowledge. This finding clashes with the popular view among scientists that relational memory requires conscious thought.

The best evidence that sleep aids memory comes from studies of automatically executed tasks, remarks neuroscientist Howard Eichenbaum of Boston University. These tasks include detecting the orientation of diagonal bars (SN: 12/2/00, p. 358: Certain memories may rest on a good sleep).

Ellenbogen’s team may have uncovered an unconsciously controlled form of sleep-enhanced relational memory, Eichenbaum says.

Bruce Bower has written about the behavioral sciences for Science News since 1984. He writes about psychology, anthropology, archaeology and mental health issues.

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