Sleep, Macbeth observed, "knits up the ravel'd sleave of care." Certainly lack of sleep knits brows. Some 25 million Americans suffer from insomnia, many to the point of seeking therapy and dangerous medication. Dr. Elliot D. Weitzman, chairman of the Department of Neurology at Montefiore Hospital in New York, has dedicated most of his career to understanding sleep and helping patients get enough of it. A native of Newark, Weitzman graduated from the University of Chicago School of Medicine in 1955—two years after one of his professors, Nathaniel Kleitman, and Weitzman's classmate Eugene Aserinsky discovered rapid eye movement (REM), or dream, sleep. That breakthrough propelled Weitzman into the field, and much of his research at Montefiore's Laboratory of Human Chronophysiology has focused on the way automatic clock-like rhythms in the body govern sleeping and waking. Weitzman, now 52, discussed with Eric Levin of PEOPLE the lessons that are emerging from his studies.

How do our "body clocks" work?

We don't know exactly, but somewhere in the brain there seem to be tiny "oscillators" that govern the timing of many metabolic activities. These daily rhythms complete one full cycle every 24 to 25 hours.

Would you give an example?

Body temperature is a good indicator. During a 24-hour day, a normal healthy person's temperature will fluctuate by as much as three degrees. The low point is normally reached in the middle of the night while we are sleeping. In most people, the temperature begins dropping to that low after 10 p.m., and corresponds to the point in our sleep-wake cycle when we are most likely to fall asleep. In a small percentage of people, the internal cycle is out of phase with society. They don't fall asleep until 3 or 4 a.m. Not understanding their rhythms, they try to go to sleep at a normal hour—and can't.

What is the treatment?

It's called chronotherapy. It has worked in all patients we've tried it with, but the regimen must be followed exactly. Basically it consists of going to bed three hours later every night for a week. This steadily advances their bedtime from about 4:30 a.m. at the start to 7:30 a.m. and finally to 11:30 p.m. at the end. Once there, these patients are able to hold it if they discipline themselves.

Is all insomnia "body-clock" related?

No. Insomnia is a symptom, not a disease, and many things can underlie it. Most often some event in your life has made you angry, frightened, sad, excited. Sleep is a delicate state, and daytime concerns intrude into it. If it only happens a couple of nights a month it's no big deal. Usually it goes away with the external problem.

Are there other, more persistent forms of insomnia?

Another category is related to severe psychopathology, such as manic-depression. You treat the depression, not the insomnia, and fortunately there are now useful drugs for that. More prevalent are the insomnias stemming from chronic neuroses, persistent obsessions, anxieties, anger, phobias and coping problems. After an interview we usually suggest psychotherapy to these people. They often respond, "Yeah, I knew it all along." Coming to us is an acceptable way of getting into the medical system. They rationalize, "I've got a sleep problem. It's not psychological."

Can common insomnia be prevented?

Not entirely. But by following a few simple rules, which we call "sleep hygiene," you increase the odds of getting a good night's sleep. The basic rule is go to bed the same time every night—for typical adults, between 11 p.m. and midnight. This organizes your sleep and keeps you in sync with your circadian—or daily—biological rhythms. Second, don't linger in bed in the morning. An extra 15 minutes or half hour after the alarm goes off is nothing to worry about, but don't let yourself doze the whole morning away. You will have trouble sleeping at night. Many housewives have this problem.

Anything else?

Try to avoid long evening naps. Some men, for example, will sleep in front of the TV until their wives wake them just before bedtime. Then they wonder why they wake up in the middle of the night. I tell them, "Look, you've already slept three hours. Your body only needs four or five more." That kind of nap can also raise psychological issues. It can be a way of withdrawing, controlling the family or a passive release of hostility.

Are you against all naps?

Not necessarily. Most of the world's population takes daily siestas, usually after lunch. Older people frequently take naps, and we've studied some of them in the lab. But the fact is most people spend a third of their time sleeping—whether in one long stretch or several shorter ones.

Do sleep requirements change with age?

Definitely. Newborn babies sleep 16 hours a day. Adolescents sleep very deeply—as their parents know—for eight to nine hours straight. Seven to eight hours is normal for adults. Contrary to popular belief, older people don't need more sleep. After about age 55 nightly sleep requirements taper off 10 to 15 percent.

What is the minimum amount of sleep a normal adult should get?

There's no such thing as a minimum that fits all people. The range for adults runs from four to five hours a night up to seven to eight; but the four-to-five-hour sleepers represent only about one percent of the adult population.

Do some people never sleep?

There are claims in the literature, but I've never seen such a person.

Normally, how long does it take people to fall asleep?

Sleep latency—the time between turning off the light and falling asleep—is less than 15 minutes. In fact, it's usually six to eight minutes. Curiously, most people overestimate how long it takes them to fall asleep.

Does that mean some professed insomniacs may actually be sleeping well?

Yes. The condition is called pseudoinsomnia. In our sleep lab, we've asked patients how long it took to fall asleep and been told, "At least an hour." But from brain wave readings, we know they fell asleep in five to 10 minutes. Some of these patients wake up after six or seven hours and insist they haven't slept at all.

Is it possible to catch up on lost sleep?

Not the way most people think. If you stay up all night and go to bed at 10 a.m., you may think you'll sleep eight hours, but you won't because your circadian rhythm has moved into the activity phase. The amount of time we sleep has less to do with how long we've been up than with when in our sleep-wake cycle we go to bed. That's an important point for jet-lag sufferers to understand. When you try to sleep out of phase with your body clock, your sleep will be somewhat fitful.

What's the answer to insomnia?

Don't be obsessed about the fact you aren't sleeping. You may feel a little tired the next day, but nothing serious is going to happen to you. Read, knit—do those little things that you never seem to get around to. But don't worry and, above all, don't start taking sleeping pills.

Why no sleeping pills?

They can lead to chronic and serious drug-dependency insomnia. The pills, especially barbiturates and Doriden, actually disturb sleep.

What is the greatest unanswered question concerning sleep?

Why we do it. No one knows precisely the purpose of sleep.

Doesn't it involve rejuvenation?

There's no evidence. I mean, we do a lot of things when we sleep—secrete growth hormones, for instance. But that could be done at some other time. Rejuvenation? The body may rest, but the brain does not. In fact during REM, or dream, sleep there's more brain activity than when we're wide awake.

So why do we feel refreshed after a night's sleep?

Because the biological rhythm has come that way If you stayed up all night, at two the next afternoon you'd feel better than you did at 5 a.m. If we understood what brain mechanism makes it happen, perhaps we could eliminate the need for sleep. We could say, "Here, take this pill and feel fine all the time. You don't have to sleep."

Will that ever happen?

No organism I'm aware of has evolved itself out of the sleep process. We live on a cyclic planet—that's part of the reason. Also I suspect the need for sleep is such an integral part of our genetic makeup that if you eliminate it, you eliminate life itself.