melatonin

noun Physiology.

1. a hormone secreted by the pineal gland in inverse proportion to the amount of light received by the retina, important in the regulation of biorhythms: in amphibians, it causes a lightening of the skin.

noun

1. the hormone-like secretion of the pineal gland, causing skin colour changes in some animals and thought to be involved in reproductive function

n.1958, from Greek melas “black, dark” (see melanin) + ending from serotonin. So called because its secretion is inhibited by sunlight. n.

1. A hormone derived from serotonin and produced by the pineal gland that stimulates color change in the epidermis of amphibians and reptiles and that is believed to influence estrus in mammals.

1. A hormone produced in the pineal gland that plays a role in regulating biological rhythms, including sleep and reproductive cycles. In many animals, melatonin also regulates the physiological effects that occur in response to seasonal changes, such as the growth of a winter coat of fur. Chemical formula: C13H16N2O2.