According to Canadian scientists there is no evidence to support the belief that melatonin has any effect in the treatment of sleep disorders.
Neither is it effective, they say, in treating sleep disorders associated with jet lag and shiftwork.
The researchers also found that while melatonin is safe over the short term, more studies are needed to determine its long-term safety.
According to the researchers as many as 20 percent of Americans suffer from sleep problems, and melatonin is often recommended as a natural nightcap in particular for relieving jet lag.
Melatonin is a hormone, which occurs naturally in the body and is produced by the pineal gland in the brain.
Previous studies have shown that levels of melatonin rise at night and fall in the morning.
It is available in over-the-counter supplements and claims to help regulate the body's daily rhythms; many shift workers and air travellers take it to improve their sleep patterns.
But scientists at the University of Alberta in Edmonton Canada found it had no significant impact.
Study leader Nina Buscemi, a research associate at the University of Alberta, says their results did not provide any evidence that supports the claim that melatonin is effective in alleviating sleep disturbance in jet lag.
Buscemi and her team looked at a number of trials in order to assess the impact of melatonin on sleep disorders caused by medical problems or lack of sleep due to working shifts or to air travel.
They found that in six randomised controlled trials with 97 participants there was no evidence that melatonin had an effect on sleep onset latency in people with secondary sleep disorders.
In nine randomised controlled trials with 427 participants there was no evidence that melatonin had an effect on sleep onset latency in people who had sleep disorders accompanying sleep restriction.
While another 17 randomised controlled trials with 651 participants showed no evidence of adverse effects of melatonin over a period of three months or less.
The report is published online by the British Medical Journal.