Melatonin, 5-methoxy-N-acetyltryptamine, is a hormone found in all living creatures from algae to humans, at levels that vary in a daily cycle. It plays a role in the regulation of the circadian rhythm 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 extremely powerful antioxidant with a particular role in the protection of nuclear and mitochondrial DNA.

Products containing either or both of isolated or synthesized melatonin have been available as a health supplement in the United States since 1993, and met with good consumer acceptance and enthusiasm. However, over-the-counter sales remain illegal in many other countries including some members of the European Union and New Zealand, and the U.S. Postal Service lists melatonin among items prohibited by Germany.