Melatonin is produced by the pineal gland, a little cone-shaped structure in the brain that releases the hormone into the bloodstream. Darkness stimulates the pineal gland and causes it to produce more melatonin, which is why some call it the "hormone of darkness." Light puts the brakes on production.

Melatonin is also present in some plant and animal foods, although in very small amounts. Bananas have it, but you’d have to eat 40 at one sitting to get one milligram of melatonin. Supplements are the only source of significant concentrations.

When scientists isolated this hormone of darkness, they naturally began to wonder whether it could change sleep patterns. Today, melatonin is widely available as a supplement, and while some researchers think it may have many other properties, the most widely accepted use is for sleep- related problems.

When taken in proper doses at the appropriate time of day, this hormone can indeed increase sleepiness and help you fall asleep, says Irina Zhdanova, M.D., Ph.D., principal research scientist for the brain and cognitive sciences department at the Massachusetts Institute of Technology in Cambridge. Studies have shown that people who take melatonin may fall asleep faster, are less susceptible to awakening during the night, and feel that their sleep is more restful.

Supplementing with melatonin is not like taking a sleeping pill, however. Instead, it nudges you toward sleep by promoting general relaxation.

Whether you’re traveling east from San Francisco to Boston or west from L.A. to Singapore, you’re probably going to lose some sleep. You are also likely to suffer other symptoms of jet lag, such as headache, irritability, and poor concentration.

Supplementing with melatonin appears to alleviate these symptoms. It fights jet lag in two ways, says Dr. Zhdanova. First, it helps you fall asleep. Second, it can reset your body clock, either forward or back, to help you adapt to a new time zone.

When you take melatonin in the afternoon, it tends to advance the body clock. In other words, if you take it at 3:00 or 4:00 p.m., you’re likely to feel ready for bed earlier than you usually would. When you take it in the early morning—say, 3:00 or 4:00 a.m.—it delays the body clock so you’re willing to stay awake longer than you normally would.

Because of these effects, melatonin has been studied as a sleep aid for shift workers, including those who rotate between day and night shifts. Most of us find it hard to stay alert at night, even if we sleep during the day. Night workers have an especially tough time and seldom adapt completely because they don’t get the time cues they need from daylight.

The question of whether melatonin can help shift workers is not an easy one to answer, says Dr. Zhdanova. Individuals respond differently to melatonin. Even if they time it to help them adjust to new work schedules, it may actually make some workers more groggy and less alert as they move from a day shift to a night shift, then back again.

Apart from its clock-adjusting effects, some researchers say that melatonin can help you "grow younger" and restore vitality to a tired sex life. There’s little evidence to support either of these claims, says Richard L. Sprott, Ph.D., executive director of the Ellison Medical Foundation, an organization that funds research on the biology of aging, in Bethesda, Maryland.

How about claims that melatonin helps you live longer? Animal studies indicate that declining levels of melatonin may be associated with accelerated aging, but that decline could be a result, not a cause.

"The idea of living as long as possible is a very tempting idea," says Dr. Zhdanova, "but the few studies done with animals were inconclusive and left too many questions unanswered."

There is some evidence that melatonin may inhibit tumor growth, at least in animals. Research on the effects of melatonin on tumor growth in humans is conflicting, but most reports do show some kind of protective action. Studies continue, with the hope that some connection may be found that will guide future research, says Dr. Zhdanova.

To many people who are feeling sleep-deprived, melatonin promises a shortcut to less insomnia and deeper rest, but some researchers urge caution.

"I would prefer that people wait until more research has been done on melatonin before they decide to supplement, but I’m realistic. I know that people are taking this supplement, sometimes with positive results. I urge them to be very careful," says Dr. Zhdanova.

Since low doses are the most effective, take no more than one milligram daily. "Increasing the dose does not increase effectiveness," Dr. Zhdanova says.

As a sleep aid, take melatonin a half-hour before bedtime. An ill-timed or larger dose could actually hamper sleep.

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