Although not licensed as a drug, melatonin is widely sold as a nutritional supplement in the USA for its purported sleep-promoting and antiageing properties. In this article, we provide some guidelines for its use in sleep disorders medicine. In brief, melatonin appears to promote sleep by producing corrective circadian phase shifts, thereby improving the alignment of the endogenous sleep propensity rhythm with the desired sleep schedule. Melatonin may also have a direct soporific effect, especially when administered during the day. We suggest that the direct soporific action results from the release of accumulated sleep drive by melatonin's attenuation of the circadian alerting signal. Melatonin has not been proven safe by the usual clinical trial criteria, but to date no catastrophes have been related to its use. Also, there is little information about the safety and efficacy of chronic administration.