Melatonin, also known chemically as N-acetyl-5-methoxytryptamine, is a naturally occurring compound found in animals, plants and microbes.  In animals, circulating levels of the hormone melatonin vary in a daily cycle. Many biological effects of melatonin are produced through activation of melatonin receptors while others are due to its role as a pervasive and powerful antioxidant.
In mammals, melatonin is secreted into the blood by the pineal gland in the brain. Known as the "hormone of darkness" it is secreted in darkness in both day-active and night-active animals.
No food has been found to elevate plasma melatonin levels in humans.
Products containing melatonin have been available over-the-counter as a dietary supplement in the United States since the mid-1990s.
Melatonin has been studied for the treatment of cancer, immune disorders, cardiovascular diseases, depression, seasonal affective disorder (SAD) circadian rhythm sleep disorders, and sexual dysfunction. Basic research also indicates that melatonin may play a significant role in modulating the effects of drug abuse such as cocaine.
Melatonin appears to cause very few side effects in the short term, up to three months, when healthy people take it at low doses.*

*Excerpt from Wikipedia