Melatonin is a hormone secreted by the pineal gland in the brain when light levels drop at night. The release of melatonin stimulates healthy sleep and regulates the body's circadian rhythms. Disturbances in melatonin levels are implicated in sleep and mood disorders such as seasonal affective disorder (SAD). In environmental illnesses there are known to be endocrine disturbances that may affect the normal release of melatonin. Supplements are widely available in tablet, capsule and liquid forms at health food stores in the US and over the internet in the UK and other countries.

(from iHerb.com)