A hormone derived from serotonin and secreted by the pineal gland and retinas of vertebrates. Melatonin secretion by the pineal is linked to the dark-light cycle of the organism’s environment, being greatest at night and lowest by day. Production is regulated by signals from the suprachiasmatic nucleus of the hypothalamus, which itself receives input from the retina (see biological clock). The hormone is involved in regulating certain diurnal and seasonal changes in the body, such as the reproductive cycle in seasonally breeding animals. Melatonin also controls pigmentation changes; it triggers aggregation of the pigment melanin into melanophores in the skin, causing the skin to turn pale. It is used as a drug to treat sleep disorders and symptoms of jet lag.