melatonin

Melatonin is a hormone synthesized in both plants and animals from the amino acid L-tryptophan. In mammals, such as humans, it is secreted by the pineal gland -- and by the retina -- and modulates the body's sleep pattern (and much more). Melatonin production in the body is triggered by darkness and inhibited by light, helping the body manage its natural rhythm. It is available over the counter as a dietary supplement in the United States.