"Melatonin 
i/ˌmɛləˈtoʊnɪn/, also known chemically as N-acetyl-5-methoxytryptamine,[1] is a naturally
occurring compound found in animals, plants, and microbes.[2][3] In animals, circulating levels of the hormone melatonin vary in a daily cycle, thereby allowing the entrainment of the circadian rhythms of several biological functions."
"Many biological effects of melatonin are produced through activation of melatonin receptors,[5] while others are due to its role as a pervasive and powerful antioxidant,[6] with a particular role in the protection of nuclear and mitochondrial DNA."
"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 (diurnal) and night-active (nocturnal) animals."
"It may also be produced by a variety of peripheral cells such as bone marrow cells,[9][10] lymphocytes, and epithelial cells. Usually, the melatonin concentration in these cells is much higher than that found in the blood, but it does not seem to be regulated by the photoperiod."