Doctors of USC

Most people would not take estrogen or testosterone supplements without consulting a physician.

Yet millions of Americans indiscriminately pop pills containing another powerful hormone-melatonin. Melatonin is produced naturally in the body and replenished on a daily basis. The increased use of over-the-counter melatonin supplements has some medical researchers concerned.

"Taking large amounts of melatonin alarms me," says Cheryl Craft, Ph.D., chair of the Department of Cell and Neurobiology at the Keck School of Medicine of the University of Southern California (USC).

"There is still much that we don't know about melatonin." In the body, melatonin is secreted primarily by the pineal gland, a small, pea-sized organ hidden deep within the brain and often dubbed the "third eye" because of its ability in more primitive animals to respond to light and dark. Melatonin levels rise during the night, making the hormone a key regulator of the body's daily biological clock.

"Melatonin is the 'dark' signal that tells your body it's time to sleep," explains Craft, who is also the Mary D. Allen Professor for Vision Research at the Doheny Eye Institute. This fact has made melatonin supplements popular with travelerswho use the hormone to shift their sleep/wake cycle and overcome the effects of jet lag.

Beyond that, melatonin's precise role remains largely a mystery to scientists, although that hasn't stopped promoters of melatonin supplements-which are not regulated by the Federal Food and Drug Administration-from claiming the hormone can do everything from enhance sexual prowess to slow the aging process. Craft notes that these and other undocumented claims are backed by little or limited scientific research and, in some cases, may even be refuted by it.

For example, studies in some animals have shown that chronic treatment with high melatonin levels cause the testes to shrink and sex drive to decrease. No one knows if similar reactions can occur in humans, but Craft finds the prospect unsettling and cautions against long-term melatonin use, especially in young people.

"If people take melatonin for ten years or more, no one knows what affect it will have," she says. "They're really engaging in an unnatural experiment."