Melatonin also known chemically as N-acetyl-5-methoxytryptamine, is a naturally occurring hormone found in most animals, including humans, and some other living organisms, including algae. Circulating levels vary in a daily cycle, and melatonin is important in the regulation of the circadian rhythms of several biological functions. 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 with a particular role in the protection of nuclear and mitochondrial DNA. Products containing melatonin have been available as a dietary supplement in the United States since 1993. However, over-the-counter sales of the hormone remain illegal in many other countries, and the U.S. Postal Service lists melatonin among items prohibited by Germany.

Melatonin, 5-methoxy-N-acetyltryptamine, is a hormone found in all living creatures from algae to humans, at levels that vary in a diurnal cycle.

Many biological effects of melatonin are produced through activation of melatonin receptors, while others are due to its role as a pervasive and extremely powerful antioxidant with a particular role in the protection of nuclear and mitochondrial DNA.

Melatonin was released into the general health supplement market in the United States in 1993, and met with good consumer acceptance and enthusiasm.