Melatonin plays a key role in regulating the body's circadian rhythm. Melatonin can be helpful in regulating sleep cycles in individuals who have irregular sleep patterns.

Melatonin is produced primarily in the pineal gland but also in the bone marrow, platelets, GI tract, eyes, skin and lymphocytes. Its chemical precursor is serotonin, a neurotransmitter derived from tryptophan. Serotonin must be acetylated and methylated to form melatonin. Synthesis and production of melatonin is affected by light exposure to the eyes; serum concentrations of melatonin are typically low during the day hours and increase when dark. Since it has important effects in affecting circadian rhythms, melatonin has been reported to have significant effects on sleep-wake cycles and antioxidant function.