From Gr. melas = black, and the ending from serotonin.

Also called the “hormone of darkness”, melatonin is secreted by the pineal gland in response to darkness. It plays a crucial role in the regulation of the circadian rhythm, it’s tranquillizing effect is used in insomnia treatment.

For further insight into it’s complex and mystified physiology check out “Melatonin, the Hormone of Darkness: From Sleep Promotion to Ebola Treatment” on PubMed