Melatonin is a natural hormone produced in the brain - its main purpose is to control sleep cycles. It is not currently licenced for use in the UK, but many Britons still use it, buying it abroad or through mail order. But researchers say that byproducts of the supplement may cause harmful mood probelms, and called for people not to buy it.

Chemists William Pryor and Guissepe Squadrito of Louisiana State University's Biodynamics Institute said melatonin was usually found only in very small concentrations in the body.

Breaking down excess melatonin in the body may cause more harm than good, they said.

Dr Squadrito said: "It is our hypothesis that secondary products of melatonin have as yet unrecognised health effects."

The findings will be published in the June issue of the American Chemical Society journal Chemical Research in Toxicology.

Brain signalling chemicals

Carbonate and nitrogen dioxide in the body react with melatonin to form two chemicals that seem to resemble brain signalling chemicals.

Although the exact biological function of such chemicals is unknown, laboratory tests showed they were significant. Dr Squadrito said: "They could have an important effect on behaviour - like mood."

The team plan further studies to determine the exact effect of excess melatonin on the body.