Melatonin is a hormone secreted by the pineal gland in the brain that helps regulate other hormones and maintains the body's circadian rhythm. The circadian rhythm is an internal 24-hour time-keeping system that plays a critical role in determining when we fall asleep and when we wake up.

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.

Melatonin also helps control the timing and release of female reproductive hormones. It helps determine when menstruation begins, the frequency and duration of menstrual cycles, and when menstruation ends (menopause).

Melatonin is now a popular therapy for jet-lag and disturbances of sleep. Claims are made for improved sex-life, longevity and even lowering of blood pressure.

Melatonin also possesses antioxidant activity, and many of its proposed therapeutic or preventive uses are based on this property.