Over the years, Allan Rechtschaffen has killed a lot of rats just by keeping them awake. In his sleep laboratory at the University of Chicago, Rechtschaffen places each rat on an enclosed turntable contraption that begins spinning whenever the rodent’s brain waves suggest it is beginning to nod off-forcing the rodent to keep moving so that it doesn’t bump into a wall. After about a week of enforced consciousness, the rat begins showing some signs of strain. Odd lesions break out on its tail and paws. It becomes irritable. Its body temperature drops even as it attempts to make itself warmer than usual. It eats twice as much food as normal but loses 10 to 15 percent of its body weight. After about 17 days of sleeplessness, the rat dies. What kills it "We don’t know," says Rechtschaffen. Thus it goes in the science of sleep. Rats can last about 16 days without eating, suggesting that sleep is nearly as vital to life as is food. Yet scientists are far from answering the seemingly simple question of what, exactly, sleep is good for. Of course, there’s no shortage of hypotheses; insomniacs hoping for some shut-eye might do well to count sleep theories instead of sheep. Many of the most popular theories are extensions of common-sense propositions from human experience. Since we feel rested after sleep, some researchers argue-that sleep must be for rest. Harold Zepelin, professor emeritus in psychology at Michigan’s Oakland University, regards sleep as a period of mandatory energy conservation. "We can’t afford to be active 24 hours per day," syas Zepelin, so evolution dictated this daily period of hibernation. (Some even argue that one reason sleep evolved in humans was to keep us unconscious and out of harm’s way during the night, when we are not exactly the king of beasts.) Smaller animals such as rodents, which have high metabolisms and expend proportionately more energy to make up for the rapid loss of heat that is a geometric consequence of smallness, do tend to sleep more. Larger animals such as giraffes sleep less than five hours each day. But the energy savings from sleep in large animals are so small it is hard to see why they would sleep at all by this theory. Humans save merely 120 kilocalories a night (about the equivalent of an apple) by sleeping rather than staying awake. Moreover, even hibernating animals arouse themselves from torpor to enter sleep and then fall back into hibernation, suggesting that there is a deeper need for sleep than a mere recharging of the body’s batteries. Dennis McGinty believes part of the function of sleep is to cool off the brain. The chief of neurophysiology research at Los Angeles’s Sepulveda Veterans Hospital, McQmty points to a feedback loop in the brain that seems to trigger sleep when the brain gets too hot. When provided with a bar to increase cage temperature, rats that are kept awake jack up the heat about 10 degrees Celsius. By attempting to get warmer than usual, the rats may be hoping to trigger sleep-inducing neurons. The phenomenon also occurs in humans. "If you exercise in the extreme heat, it practically knocks you out," McGinty notes. Well-trained athletes who are able to increase their body temperature during exercise—unlike us weekend workout warriors—sleep about one hour longer than normal. In essence, a jump in body temperature activates heat-sensitive neurons to slow down the body’s metabolism—preferably by sleep—and thus cool down the brain. The body’s minimum temperature comes during the deepest sleep, typically at around 5 a.m. Though scientists are dubious about some experiments, they believe that______.

A. the body’s minimum temperature comes during the deepest sleep, typically at around 5 a.m.
B. by attempting to get warmer than usual, the rats must want to trigger sleep-inducing neurons
C. part of the function of sleep is to cool down the brain
D. hibernation in this daily period benefits animals