Many Americans in search of more satisfying slumber are buying the hormone melatonin at their local health food stores. Melatonin-containing products are marketed as dietary supplements, which can be sold without FDA's premarket review or approval.

Melatonin can actually disturb sleep cycles in doses above 1 mg. Interactions with other drugs are unknown. The response to melatonin appears to vary so widely in people, that it will be a long time before its benefits and risks can be fully understood.

It should be stressed that melatonin is a powerful hormone that can have major effects -- many still unknown -- on all parts of the body. High doses of melatonin have been associated with mental impairment, drowsiness, severe headaches, and nightmares. It may increase the risk for seizures in children with existing neurologic disorders. Daily doses of less than 1 mg rarely cause side effect.

"Public fervor for melatonin runs far ahead of the scientific evidence to support it," states an article about the NIH workshop in the Journal of the American Medical Association, referring to the reported $200 million to $350 million U.S. market for the hormone.

"People are taking melatonin and we are trying to figure out what it does," said one researcher who attended the NIH meeting. "We are going about it backward."

NIH sleep expert James Kiley, M.D., agrees that many questions about the supplement remain unanswered: "We need some research to address the concerns about melatonin and its safety and efficacy."