Why Do We Sleep?

Any second grader knows why humans need food and water. The logic behind sex becomes obvious with a quick lesson on birds and bees.

But even the most gifted scientist on the planet cannot explain why people sleep.

''It may be the biggest open question in biology,'' said Dr. Allan Rechtschaffen, a sleep expert and a professor emeritus at the University of Chicago.

Scientists' ignorance about one of life's most basic activities is not for lack of trying.

Fifty years of sleep research have ruled out some possibilities and yielded a variety of intriguing leads.

But the researchers who bragged, at a conference in the early 1970's, that the secret of sleep would be theirs by the millennium have had to revise their estimates.

''We were too optimistic,'' said Dr. Michel Jouvet, a professor emeritus at Claude Bernard University in Lyon, France, and a member of the French Academy of Sciences who attended that long-ago meeting. ''The brain is more complicated than we thought.''

Still, scientists know far more than they once did.

The discovery of rapid eye movement, or REM, sleep in 1953 awakened scientists to the realization that sleep was not ''a simple turning off of the brain,'' but an active, organized physiological process, said Dr. Jerome Siegel, a professor of psychiatry and biobehavioral sciences at the University of California at Los Angeles.

Five decades later, few researchers would dispute that sleep serves some critical -- if unknown -- biological purpose.

All mammals, birds and reptiles engage in some form of sleep, Dr. Rechtschaffen noted in a 1998 paper, even if they do it perched on a tree branch or, like the dolphin, while swimming, with one half of the brain at a time. Sleep has also endured through the eons, despite the fact that it interferes with other survival-enhancing activities.

''While we sleep, we do not procreate, protect or nurture the young, gather food, earn money, write papers, et cetera,'' Dr. Rechtschaffen wrote.

Equally telling is the finding that when humans and other animals lose sleep, they proceed to make it up, paying off the ''debt'' by sleeping longer or more intensely.

Sleep deprivation over long periods appears to have serious consequences, though what they are is still debated, because it is difficult to separate the effects of lost sleep from those of stress or other factors.

''One can't remove sleep and change nothing else,'' Dr. Siegel said.

Researchers once thought that a prolonged lack of sleep produced mental illness. They now know that this is not the case, though waking subjects up every few minutes, early studies showed, made them cranky. Nor is there proof that humans have expired from a lack of sleep. But rats deprived of sleep die in two to three weeks, or in five to six weeks if they are deprived only of REM, a sleep stage in which brain activity is similar to that in waking. Whether the rats die from massive heat loss, infection or other cause is unclear.

What is it about sleep that makes it essential to life? Experts say that, despite widespread belief, it is not simply the fact that humans and other animals need rest.

''You can rest all you like and you still need sleep,'' Dr. Rechtschaffen said.

Another theory holds that sleep may serve to protect animals, by taking them out of circulation during the dangerous hours when predators roam. Yet this theory, Dr. Rechtschaffen and others point out, cannot explain why the sleep winks lost one night are made up the next or why the impact of long-term sleep deprivation is so severe.

''It's clear,'' said Dr. J. Allan Hobson, a Harvard sleep researcher, that sleep is ''not just to get you off the street and save you a few calories.'' Dr. Hobson; Dr. Robert Stickgold, also of Harvard; and other experts have argued that REM sleep helps consolidate memory and advance learning, and a number of studies have examined this premise, including two reports published in the journal Nature last month.

But other researchers, including Dr. Siegel, have challenged this theory. People who take antidepressants called monoamine oxidase inhibitors, which suppress REM sleep, do not show memory deficits, Dr. Siegel noted in a 2001 review.

Similarly, patients with brain injuries that do away with REM appear to suffer no problems in memory, Dr. Siegel said. One Israeli man, injured by shrapnel, went to law school and served as the puzzle editor for a local newspaper. Nor are the animals that spend the most time in REM -- the platypus, for example, which averages eight hours of REM each day compared with the two hours typical of humans -- known for their learning ability or powers of recall.

Dr. Hobson responded: ''It's not to say that memory depends on REM sleep. It is to say that certain aspects of mnemonic function are enhanced during REM.''

Dr. Siegel himself has waded into the mysteries of REM sleep. As in waking, most neurons in the brain fire actively during REM. The exception is nerve cells involved with the transmitter chemicals serotonin, norepinephrine and histamine, which remain inactive. It is possible, Dr. Siegel and others have suggested, that these neurons become overused, and that REM allows them to rest and regain their sensitivity.

Smaller animals, studies have found, sleep longer than large ones: a horse snoozes for 3 hours a day, a ferret close to 15. The fact that an animal's metabolic rate slows with size has led to yet another hypothesis about sleep's purpose: that it may act to repair cell damage caused by free radicals, chemicals released during the metabolic process.

The most promising theory so far, some experts believe, proposes that REM sleep plays a role in brain development. Newborns spend more time in REM than adults. Animals that spend long periods in REM are also more immature at birth.

In the meantime, the search continues. The answer, experts say, may turn out to be something obvious or something not yet dreamed of.

''There is something tremendous out there,'' Dr. Rechtschaffen said, ''and we just haven't found it.''