melatonin

mel·a·to·nin

noun \ˌmel-ə-ˈtō-nən\

Definition of MELATONIN

: a vertebrate hormone C₁₃H₁₆N₂O₂ that is derived from serotonin, is secreted by the pineal gland especially in response to darkness, and has been linked to the regulation of circadian rhythms