Melatonin is a naturally occurring substance found in all animals. It is known chemically by the name N-acetyl-5-methoxytryptamine.
Melatonin is a circulating hormone that affects the daily cycles of mammals. It is part of what helps animals and humans in the entrainment of the circadian rhythms.
What this means is that through melatonin receptors in the body, melatonin is responsible for signalling sleep and wake times. Variable levels of daylight actually affect natural melatonin production in the body. The secretion of melatonin thus helps to trigger sleep as daylight comes to a close and less light reaches the eyes.