TRAVELING between time zones or working odd hours can wreak havoc on sleep. So, too, can certain medications or health problems. Might this be ameliorated by supplements of the hormone melatonin -- thought to be involved in regulating the body's daily rhythm?

A study reviewed 15 studies in which 524 people with sleep disorders had been randomly assigned to take melatonin or a placebo for three months or less. The hormone had no measurable effect on the participants' ability to fall asleep, whether they were deprived of sleep because of jet lag or shift work or because of medical or medication problems. The latter group registered a 2% improvement in "sleep efficiency" -- the amount of time in bed spent asleep -- but the authors described this as too small a change to be important. Sleep efficiency did not improve for shift workers and people with jet lag. A separate review of 17 studies, with 651 participants, found that people who took the hormone showed no adverse effects. Who may be affected by these findings? The estimated 40 million Americans who have trouble sleeping.

Caveats: The study did not determine whether melatonin affects daytime fatigue or is safe for long-term use.

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