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The brain, like the rest of the body, runs on glucose, Belenky explains. Using computerized scanners that provide images in real time, he and his colleagues have shown that the brain's ability to use glucose drops off dramatically after being awake 24 hours, indicating a decrease in brain activity--despite the fact that there's still plenty of glucose available. The biggest drops occur in exactly those areas of the cortex that anticipate and integrate emotion and reason. After 24 hours, however, the drop-off stabilizes. "But performance doesn't level off," Belenky notes. "It continues to tank." Why? No one knows.
In addition to refueling the brain, sleep seems to detoxify it. Animals with a high metabolic rate, like field mice and bats, use a lot of calories and generate a lot of destructive molecules called free radicals. "The brain is particularly susceptible to this because neurons, by and large, don't regenerate," says Jerome Siegel, a neuroscientist at UCLA and the Veterans Affairs Medical Center in Los Angeles. Maybe sleep provides necessary downtime so that the brain can deal with all those free radicals.
Some of the most provocative sleep research doesn't have anything to do with the brain at all. A few years after researchers isolated a natural hormone they called leptin, which tells the brain that the body has enough fatty tissue, Eve Van Cauter and her colleagues at the University of Chicago began to wonder whether sleep deprivation has any effect on the amount of leptin in the blood. They soon discovered that after just a couple of days in which 12 male volunteers were allowed to get only four hours of sleep a night, their leptin levels fell sharply, signaling the brain that a lot more calories were needed. Could a hormonal imbalance, brought on by staying up too long, help tip your metabolism in favor of gaining weight? Maybe. Just last week researchers at Stanford and Wisconsin reported similar results in a study of 1,000 volunteers. But it's also true that being overweight often interferes with the quality of sleep. At any rate, "sleep is not only for the brain," says Van Cauter. "It's also for the rest of the body."
HOW MUCH IS ENOUGH?
Whatever combination of exotic or mundane things sleep turns out to be for, researchers admit they still don't know the ideal amount of it needed to keep our bodies and brains in good working order. "There's this enormous commercial push now to convince people that if they don't get eight hours of sleep a night, there's something wrong with them," Siegel says. But in fact, there's more mythology than substance to the eight-hour figure. Back in the 1980s, a survey of more than 1 million people found that those who slept more than 7 1/2 hours a night tended to die a little sooner than their more sleep-deprived counterparts. But there is a wide enough variation in the data that you can't use the results to make any blanket statements about how much any individual should sleep. Nor can you assume that you're endangering your health if you sleep longer.