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For years sleep researchers focused most of their attention on REM sleep because, frankly, it seemed more interesting--all those dreams and everything. But they kept running into blank walls. Early work that tried to link REM sleep to learning foundered when scientists discovered that their test subjects could remember long lists of new words or facts whether or not they got any REM sleep. Indeed, an Israeli man with a piece of shrapnel in his brain became famous in sleep circles for not getting any REM sleep at all. Despite that, he went to law school and seems to have no trouble handling new situations. Many investigators gave up trying to figure out what sleep was for and focused their attention on treating various sleep disorders, such as insomnia and narcolepsy.
NEW TOOLS, NEW IDEAS
Two things happened in the mid-1990s, however, that revived research into the fundamental purposes of sleep. A 1994 study by scientists at the Weizmann Institute in Rehovot, Israel, suggested that researchers had been looking at the wrong kind of memory processing. And the technology for peering inside a sleeping brain got a whole lot better.
What the Weizmann researchers found was that your ability to recognize certain patterns on a computer screen is directly tied to the amount of REM sleep you get. Such skills depend on something called procedural memory, which is needed for any task that requires repetition and practice. Remembering a fact, like the name of the first U.S. President, is an example of declarative memory, a different kind of capability that apparently is not affected by REM sleep. Says Robert Stickgold, a cognitive neuroscientist at Harvard Medical School: "We were basically naive about memory."
But that changed once scientists knew which kind of memory to study. Over the past couple of years, Stickgold has teamed up with Matthew Walker at Boston's Beth Israel Deaconess Medical Center to investigate sleep's effects on procedural memory for motor skills. They asked right-handed test subjects to type a sequence of numbers (for example, 4-1-3-2-4) with their left hand over and over again as fast as they could. No matter what time of day they learned the task, their accuracy improved 60% to 70% after six minutes of practice. When subjects who learned the sequence in the morning were retested 12 hours later, they hadn't significantly improved. But when those who learned the sequence in the evening were retested following a night's sleep, they were an extra 15% to 20% faster and 30% to 40% more accurate.
Much to the researchers' surprise, the greatest improvements appeared in those who spent the most time in the second stage of non-REM sleep. Other procedural tasks that depended more heavily on visual or perceptual ability required periods of deeper sleep or both slow-wave and REM sleep. Sometimes even just an hour of shut-eye made a big difference. Other times a full night's rest was needed. "It's probably going to turn out that different types of memory tasks need different kinds of sleep," says Stickgold.
HIDDEN TRICKS