The secrets of sleep
It's a mystery, but it clearly makes us smarter and healthier
How exactly this mental magic happens isn't clear. It does seem that some kinds of learning can take place without long snoozes. Indeed, Stickgold's group found that a 60-to-90-minute nap works as well as a full night's sleep--as long as sleepers get both REM and non-REM sleep. Indeed, recent research indicates that despite the historical interest in dreamy REM sleep, the less sexy deep sleep may also play an essential role in learning.
In one recent study, rats were allowed to explore novel objects like a golf ball mounted on a spring. Then, using tiny electrodes in the rats' brains, the researchers monitored over 100 neurons for several days. Duke University's Sidarta Ribeiro found that electrical patterns recorded as the rats first encountered the objects later got replayed over and over during the next two days, mainly during sleep. The mental reverberation was, surprisingly, most common during the slow-wave sleep. This kind of replay during sleep has also been seen in finches learning new songs and in brain scans of people learning to anticipate a pattern of flashing lights, suggesting that during sleep, the brain goes over new information again and again.
Another way to probe the sleeping brain is to use drugs that interfere with certain chemical changes that occur over the course of sleep. During slow-wave sleep, levels of a chemical called acetylcholine normally drop to low levels, but Born and his colleague Steffen Gais recently tested the effect of this chemical with the help of an Alzheimer's drug that keeps levels artificially high. The scientists gave the drug to half of a group that was taught to memorize a list of word pairs before going to sleep. After a while, everyone woke up and took the test again. People who got the drug did worse, suggesting that the normal chemical changes in slow-wave sleep aid learning.
There are sleep researchers who don't buy any of this. The University of California-Los Angeles's Jerry Siegel, an outspoken critic of the sleep-learning hypothesis, argues that many of these studies simply contradict each other. In the acetylcholine study, for example, he points out that many researchers haven't seen a link between memorizing word lists and sleep. Plus, the drug might just have taken away the normal restful effect of sleep, and fatigued people would do more poorly on any cognitive test. In addition, he notes, people on many common antidepressants get far less REM sleep than normal but don't seem to suffer big learning gaps. And regarding the mathematical insights reported in Born's study, he points out that people get many, many insights during their waking hours, not just during sleep.
Proponents of the sleep-learning theory in the end fall back on a basic fact: Sleep's total isolation from the waking world seems designed to focus attention inward, on information already obtained that needs to get organized. "It's frankly the only biological role for sleep that makes much sense for me," says Stickgold, who notes that the body can rest during quiet wakefulness and that shutting off awareness from the rest of the world is actually a pretty dangerous act that must serve some important function.
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