melatonin

noun

Etymology: From mela- + tone + -in, ultimately from Ancient Greek μέλας (mélas, “black, dark”).

1. A hormone, related to serotonin, that is secreted by the pineal gland, and stimulates colour change in the skin of reptiles, and is involved in the sleep and wake and reproductive cycles in mammals.

   “How exactly melatonin relates to sleep is still not understood.”

   “Melatonin production can be delayed or arrested by even brief flashes of bright light entering the eye, research has shown.”

2. Any material similar in its chemistry and effect to the natural hormone.