Melatonin is a hormone produced by the pineal gland mainly during the night. Its primary function is to coordinate biological rhythms. It may be used for therapy of chronobiological disorders, such as occur in jet lag, shift work, blind persons or in specific forms of insomnia. Due to its anti-gonadotropic effect, melatonin in high doses has been used as a contraceptive. In-vitro and animal studies have demonstrated cancer inhibition, immun-stimulating and rejuvenating effects, but none of these claims have been confirmed in clinical studies. Melatonin used over a short period is not associated with significant side-effects, but the tolerability of long-term administration is unknown. As the products are not always of pharmaceutical quality, one should avoid uncontrolled use of melatonin.