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By Mary Carmichael
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Like many students, Kimberly Fenn has pulled more than a few all-nighters, cramming facts into her head for the next day's exam, fighting exhaustion and gravity to keep her eyelids from closing. Her parents always told her she'd be better off with a good night's sleep. But it was only this past year, as a psychology graduate student at the University of Chicago, that Fenn learned how true that was. She tested two groups of undergraduates on their ability to learn a gibberish language. One group (the crammers) had to take a test on the same day as their training, while another group (the sleepers) were allowed a night's rest. The results were startling. The sleepers scored higher than the crammers, "like going from F's to C's," says Howard Nusbaum, Fenn's adviser and a coauthor of the study.
The findings do more than merely confirm what we (or at least our moms) already knew. They suggest that the link between sleep and the brain's higher mental processes is far stronger than researchers previously suspected. Psychologists have long known that sleep enhances basic cognitive and motor skills, and they've shown that people tend to sleep longer after heavy mental workouts, like learning to program in Basic. Fenn's study suggests that the sleeping brain plays an essential role in turning the rush of daily events into long-term memories--not mere facts and images but elaborate mental processes, like how to tie your shoelaces or prove Pythagoras' theorem.
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In her study, published last week in the journal Nature, Fenn had her undergrads learn a nonsense language. She started by playing a recording of a gibberish word ("nawn") while flashing its English equivalent ("lawn") on a screen. Then she continued to feed them other translations, like "frud" for "frog" and "snurt" for "smart." Finally, she tested them by giving them a gibberish word and asking them to figure out what the English equivalent should be. The idea was to deduce the pattern and apply it to new words. While the crammers didn't perform well, the sleeping students had a chance to consolidate the memories of their skills overnight, even if they hadn't gotten the hang of it the day before. "At night, sleep was restoring the information they'd lost earlier in the day," says Nusbaum. "It was solidifying what they had learned."
Exactly how the brain consolidates and retrieves lost memories during sleep is still something of a mystery. One theory holds that sleep allows the brain to prune away irrelevant information picked up during the day that can interfere with its ability to retrieve more important memories. An alternate theory says that sleep gives the brain access to memories it simply couldn't get to the day before. This suggests that forgotten experiences "aren't actually gone," says Fenn; they just sometimes can't be retrieved until they've made the leap to long-term memory.
Another question is whether dreaming plays a role in learning. Dan Margoliash, a University of Chicago professor, has scanned the brains of birds and found that they exercise the same neurons when they dream that they use when producing calls. By replaying their songs at night, birds may be practicing their skills. "Do I think there's replay in humans?" he says. "Absolutely." This nightly "replay" could be our way of solidifying memories, and it may help answer the age-old question of why we dream. Scientists won't know for sure until they've had a chance to test humans the way Margoliash has tested birds. There are other questions to clear up, too. Do chronically sleep-deprived students perform poorly on tests like Fenn's? Harvard researchers found last year that a quick doze helps people perform simple tasks, but would power naps help Fenn's students? Until more studies are done, it might be a good idea to ask Mom.
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