N-Acetyl-5-methoxytryptamine; a substance formed by the mammalian pineal gland, which appears to depress gonadal function in mammals and causes contraction of amphibian melanophores; a precursor is serotonin. Melatonin is rapidly metabolized and is taken up by all tissues. It is involved in circadian rhythms.
Melatonin secretion is linked to both sleep-wakefulness and light-dark cycles. Ocular perception that ambient light is dimming has been shown to promote increased secretion of melatonin by the pineal gland through neural pathways involving the hypothalamus. Serum levels increase tenfold just before sleep and peak around midnight. Twenty-four-hour secretion is higher in winter than in summer. The decline of melatonin secretion with age has been blamed for the tendency to insomnia in the elderly. Because melatonin acts as an antioxidant in counteracting free radicals, it has been promoted as a means of delaying aging and preventing cancer, heart disease, and Alzheimer dementia. It has also been proposed as an antidepressant because serotonin (5-hydroxytryptamine), the metabolism of which is known to be disordered in clinical depression, is a chemical precursor of melatonin. Adequately controlled, large-scale studies of the efficacy, safety, and optimal dosage of melatonin are lacking. There is experimental evidence that long-term administration can reset the circadian pacemaker. Several studies support the effectiveness of melatonin in preventing or reducing jet lag, particularly for travelers flying east across five or more time zones. The direct soporific effect of melatonin varies widely from person to person. Limited studies suggest that it may increase the duration of restful nighttime sleep in the elderly. High doses of melatonin result in prolonged elevation of serum melatonin level and increased production of prolactin by the pituitary gland. Unlike most hormones, melatonin is readily absorbed from the digestive tract and is a component of some foods. Hence therapeutic formulations are not subject to federal drug regulations or purity standards. Testing of commercially available preparations of melatonin has indicated both variation in potency and the presence of possibly harmful contaminants.
Reference: Stedman's Medical Dictionary