Melatonin is a hormone produced in the brain by the pineal gland from the amino acid tryptophan. The synthesis and release of melatonin are stimulated by darkness and suppressed by light, suggesting the involvement of melatonin in circadian rhythm and regulation of diverse body functions. Levels of melatonin in the blood are highest prior to bedtime. Synthetic melatonin supplements have been used for a variety of medical conditions, most notably for disorders related to sleep. Melatonin possesses antioxidant activity, and many of its proposed therapeutic or preventive uses are based on this property. New drugs that block the effects of melatonin are in development, such as BMS-214778 or luzindole, and may have uses in various disorders.