There is no evidence the hormone melatonin is effective in preventing jet lag or treating sleep disorders, research has found.
Jet lag can disrupt sleep patterns badly
Melatonin plays a role in controlling daily body rhythms, and has become popular in supplement form to treat sleep problems.
The University of Alberta study suggests there is little evidence to support this.
But UK experts have challenged the British Medical Journal study.
Melatonin is produced naturally by the pineal gland in the brain. Research has shown that levels rise at night and fall in the morning.
The researchers looked at the use of melatonin to treat people with 'secondary' sleep problems, often caused by medical or psychological conditions, or substance misuse
They also assessed whether the hormone could help people with disturbed or restricted sleep, such as shift workers, or those with jet lag.
In total, they examined data from 16 trials including more than 500 people.
Melatonin had no significant impact - either on increasing amount of sleep, or reducing the time taken to fall asleep - among people with disturbed or restricted sleep.
It did increase amount of sleep among people with secondary sleep problems.
But the effect was so small - less than 10 minutes extra sleep in an eight-hour period spent in bed - that the researchers dismissed it as clinically unimportant.
The researchers found no evidence to suggest that use of the hormone posed any safety risk in the short term.
But they said further studies were needed to determine its long-term safety.
Dr Derk-Jan Dijk, a sleep expert at the University of Surrey, told the BBC News website he was surprised by the findings.
He said several studies had suggested that melatonin was an effective treatment for jet lag - although there was evidence to suggest that its effectiveness varied depending on the time of day it was taken.
Dr Dijk said: "We know quite a bit about the physiological role of melatonin in the regulation of the sleep/wake cycle, and tightly controlled laboratory studies have clearly indicated that it can facilitate sleep at the 'wrong' time of day."
Dr Dijk added that sleep-inducing drugs, or hypnotics, had been developed which act on the melatonin receptors within cells.
Dr Adrian Williams, director of the Sleep Disorders Centre, at St Thomas' Hospital, London, agreed that research backed the use of melatonin to treat jet lag.
"I use it myself to treat body clock problems, and I believe it has benefit for some sleep conditions," he said.
No Holy Grail
However, Dr Andrew Cummin, of the Sleep Laboratory at London's Charing Cross Hospital, welcomed the research.
He said: "At last the myth of melatonin as a cure-all for sleep problems has been exploded. Sadly, melatonin is not the Holy Grail we have all been seeking.
"Blind faith may drive many to continue with what has been seen as a natural remedy for jet lag and other sleep problems. But melatonin is not without risk.
"In men it decreases semen quality and women should be aware that it affects the reproductive system of animals. The blunt truth is that, as yet, we have no quick fix."
Melatonin is not a licensed medicine in the UK, but it is available on prescription.