hormone secreted from the pineal gland
Note 1 to entry: Levels of melatonin in the pineal gland, blood, saliva, and urine fluctuate in a circadian rhythm, higher at night and lower in the daytime, following the biological clock that is itself set naturally by day/night photoperiods. Light at night reduces melatonin, while decoupling the biological and natural (or artificially induced) photoperiods causes disruption of sleep/wake cycles (e.g. jet lag).
Note 2 to entry: This entry was numbered 17-762 in CIE S 017:2011.