Dr. Lewy, director of the sleep lab at Oregon, said, ''Totally blind people offer a natural way to study the human body clock, without the interference of light cues.''

In an earlier study, the researchers tried using a 5-milligram dose of melatonin without success. ''We got some shifting of rhythms, but we weren't able to establish a clear 24-hour cycle,'' Dr. Sack said.

A few otherinvestigators have reported single cases in which melatonin corrected free-running sleep-wake cycles in blind people. And Dr. Stevenson, now a research psychologist at NASA Ames Research Center in Mountainview, Calif., said he began taking melatonin before bedtime in 1987, and now sleeps normally. But the new study compared melatonin with placebo pills in a group of subjects.

Melatonin is secreted by the pineal gland -- a tiny structure deep in the brain -- and helps regulate the body's biological clock. In sighted people, melatonin secretion is synchronized with the 24-hour cycle of daylight and darkness, turning on at nightfall, and continuing for about 12 hours. But in the absence of light, most people's body clocks run on a cycle slightly longer than 24 hours, Dr. Sack said.

The hormone is also sold over the counter as a nutritional supplement, usually in 3-milligram tablets. Taken in the afternoon, it shifts the body's clock earlier, tricking it into thinking dusk has already fallen. Taken in the morning, however, it delays the clock, as if dawn had not yet arrived.

Scientists still know little about the effects of melatonin at different dosages: In sleep studies, researchers have used doses as small as .3 milligrams and as large as 75 milligrams or higher. Dr. Sack said for some blind people 10 milligrams may be more than is needed. He and his colleagues are conducting further research to see if dosages between 5 and 10 milligrams are equally effective.

In the study, the subjects, who did not know which treatment they were receiving, took melatonin an hour before bedtime for a period of weeks, then were switched to a dummy pill, or vice versa. The length of time it took for the melatonin to shift rhythms back to normal varied from person to person. ''Without knowledge of a person's circadian phase,'' Dr. Sack and his colleagues wrote, ''it may be difficult to know the best day for initiating melatonin treatment; thus entrainment may not occur for weeks or months.''