Sleep disorders lead to impaired functioning or excessive sleepiness.

There's no evidence melatonin supplements help people overcome sleep disorders like jet lag, says a new study from the University of Alberta.

Melatonin is a hormone produced in the brain's pineal gland that affects our internal clock and sleep cycles, leading people to assume the natural supplement could work as a an alternative to sleeping pills.

Nina Buscemi of the University of Alberta's department of pediatrics led a review of 32 melatonin studies involving more than 500 people.

The studies tested melatonin in two groups: those with sleep disorders caused by jet lag or shift work, and those with secondary sleep disorders brought on by medical or psychological problems such as depression.

"Our results do not provide evidence that melatonin is effective in alleviating sleep disturbance in jet lag," the review's authors wrote in the Feb. 10 online issue of the British Medical Journal.

Melatonin supplements did appear to be safe to take in the short term, but more research is needed to see whether the supplement can be used safely for longer periods, the team said.

Melatonin also increased the amount of sleep among people with secondary sleep problems. But it only added 10 minutes of extra shut-eye within an eight-hour period spent in bed, which the researchers dismissed as "clinically unimportant."

They note melatonin formulations vary in quality, dosage and how long it was administered, which may explain the range of effects reported in the studies reviewed.

About 20 per cent of Americans suffer from sleep disorder that lead to impaired functioning or excessive sleepiness, according to the study.