"A lot of people take melatonin for jet lag and shift work, but we found no good evidence to justify the use of melatonin in this context," said study author Dr. Terry Klassen, director of the Evidence-Based Practice Center at the University of Alberta in Edmonton, Canada.

The study, which was funded by the U.S. National Center for Complementary and Alternative Medicine, was published online by the Agency for Healthcare Research and Quality (AHRQ). It is the most comprehensive look at the science behind the claims that melatonin is a beneficial sleep aid, Klassen said.

"There has been a lot of emphasis on individual studies rather than the overall picture, but when you put it all together, it tells a more compelling story," he said.

The researchers found the quality of the studies varied, with those that looked at the effects of melatonin on normal sleepers generally of lower quality than those studies that looked at people with sleep problems. Further, Klassen said, all the studies tended to be small and of short duration, with an average length of a few months, so that the long-term effects of melatonin could not be assessed.

The authors also noted that in the studies assessing the benefits of melatonin for normal sleepers, "there was considerable evidence of possible publication bias in the selection of the studies analyzed," they wrote. "We found a greater number of studies reporting positive results compared to negative results."

The one area where melatonin did seem to work was with a group of people with a condition called "delayed sleep phase syndrome," which means that otherwise healthy people have a biological quirk that prevents a normal sleep cycle. For these people, Klassen said, taking melatonin appeared to improve their sleep.

"Melatonin helped to reset their biological clock," he said, so that on average it reduced the time it took them to fall asleep by about a half-hour.

Klassen said that although many people swear that melatonin works for them when they're suffering from jet lag, the science simply doesn't back up these claims.

"It could be a placebo effect, or the fact that sleep disorders vary, and that someone would get better anyway," he said.

According to the National Institutes of Health, approximately 40 million Americans suffer from chronic, long-term sleep disorders each year, and an additional 20 million experience occasional sleeping problems. The most common sleep disorders are insomnia, sleep apnea, restless leg syndrome (particularly among elderly people), and narcolepsy, a condition in which people suddenly fall asleep for short periods of time even if they are well-rested.

Melatonin is a hormone found in the brain that helps the body's sleeping and waking schedule. Typically, melatonin levels rise in the evening, causing sleepiness, and are lower in the morning to induce waking.

"When people go into stores, if a bottle of melatonin has the word 'sleep' on it, there is an implied medical claim that they're going to sleep better, and this report doesn't support these implied benefits. It's a very good study," said Dr. Meir Kryger, director of the Sleep Disorder Clinic at St. Boniface Hospital Research Center at Winnipeg's University of Manitoba in Canada and author of A Woman's Guide to Sleep Disorders.

This is exactly why evidence-based studies are so important, Klassen said.

"This is a huge issue, and an area where we should be spending money to fund rigorous testing of these claims," he said.

"Sleep disorders can affect a person's quality of life and job performance, which can translate into decreased productivity, motor vehicle and industrial accidents, and even medical errors," AHRQ Director Dr. Carolyn M. Clancy said in a statement.

More information

Get tips on sleep aids from the National Sleep Foundation.