Sleeper Effects: Slumber may fortify memory, stir insight

There’s nothing like a good night’s sleep to get some serious thinking done. That, at least, is the theme of two new investigations, one conducted with rodents and the other with people.

Rats permitted to explore novel objects display distinctive activity throughout much of their brains. That activity reappears—even more strongly than originally—during a stage of slumber called slow-wave sleep, say neuroscientist Sidarta Ribeiro of Duke University Medical Center in Durham, N.C., and his colleagues.

A reprise of waking neural activity during slow-wave sleep—the longest sleep stage in rats and people—promotes recall of novel experiences, the scientists propose. Then, neural changes crucial for memory storage occur during rapid eye movement (REM) sleep, in their view.

“These two phases of sleep play separate, complementary roles in memory,” Ribeiro says. The new findings appear in the January Public Library of Science Biology, an online journal.

Ribeiro’s team implanted electrodes in the brains of five rats to measure the activity of 59 to 159 neurons per animal. The electrodes were placed in the cerebral cortex, the hippocampus, the putamen, and the thalamus—regions that participate in sensory processing and memory formation.

After recording the animals’ brain activity for 2 days, the researchers gave the rats access to four novel items: a golf ball mounted on a spring, a small brush, a stick with pins attached, and a tube that dispensed pieces of cereal.

Exploration of these objects elicited distinctive electrical activity throughout the rats’ brains. This activity reappeared more strongly during slow-wave sleep over the next 2 days.

Slow-wave sleep orchestrates the recall and amplification of recent memories, Ribeiro proposes. Earlier research by Ribeiro and others indicated that REM sleep triggers certain genes to make proteins that groom brain cells for memory storage. Scientists have yet to determine whether rats that show activity reprises during slow-wave sleep actually display improved memory for the novel items.

A second study, in the Jan. 22 Nature, suggests that sleep’s preservative effects on memory foster problem-solving insights.

Neurologist Jan Born of the University of Lübeck in Germany and his coworkers studied 66 volunteers who transformed a string of eight digits into a seven-digit sequence, using simple rules for converting each pair of digits in the initial string into a digit in a final string. The goal was to identify the last number in the new sequence as quickly as possible.

Participants improved either by calculating whole sequences faster or by realizing that the testers had chosen numbers such that the second number calculated was always the same as the final one in the new sequence.

None of the participants recognized that shortcut on initial trials. After a night’s sleep, though, 13 of 22 volunteers solved the task much more quickly than they had before and described the shortcut to the researchers. That realization occurred in only 5 of 22 people who were retested after 8 hours of daytime wakefulness and in 5 of 22 who were retested after 8 hours of nighttime wakefulness.

“This is the first study to demonstrate rigorously that sleep can influence insight in problem solving,” comments neuroscientist Pierre Maquet of the University of Liège in Belgium.

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