The sleep-regulating hormone melatonin cannot rapidly reset a person's body clock when traveling through time zones, according to a new report.

Results of a study of baboons contradicts the popular notion that melatonin, produced by the pineal gland at the base of the brain and sold in a synthetic form in health-food stores, can help a person to overcome the effects of jet lag and aging.

"We found that melatonin, given as is currently prescribed for conditions such as jet lag, does not shift the circadian clock," Dr. Scott A. Rivkees, the study's lead author, explained in a prepared statement.

However, the study findings suggest that melatonin may promote sleep when taken at night, according to the report in the Journal of Clinical Endocrinology and Metabolism.

In an interview with Reuters Health, Rivkees said the hormone's perceived ability to relieve jet lag might relate to its sleep-promoting ability, since flying through multiple time zones can cause sleep problems.

Circadian rhythms are daily cycles based on a biological clock in the brain. Travel through time zones or working a night shift may disrupt these natural cycles, leading to fatigue and an impaired ability to function.

Rivkees and co-author Haiping Hao from Yale University School of Medicine in New Haven, Connecticut, measured circadian rhythms in three baboons, which have similar patterns of sleep as humans. They gave the animals different doses of melatonin at various times of the day over a 2-week period.

The investigators monitored the activity patterns of baboons in a constantly dim room, and observed whether the melatonin treatments shifted circadian rhythms by continuously monitoring the animals for another 10 to 14 days.

Melatonin did not shift circadian rhythms but it did induce sleep when given at night. The same dose of melatonin given during the day did not cause sleep.

Previous studies have also shown that melatonin can induce sleep. Recently, researchers reported that the hormone may restore normal circadian rhythms in the blind.

(From Journal of Clinical Endocrinology and Metabolism October 2000.)