<prompt> Melatonin, plays an impoant role in sleep-wake cycle, is secreted by:Pineal glandThyroid glandAnt. pituitaryPancreas  <response>: Melatonin is an endogenous hormone that is normally secreted by the pineal gland and is believed to be involved with the circadian sleep-wake cycle. Melatonin synthesis and secretion are increased during the dark period of the day and maintained at a low level during daylight hours. This diurnal variation in secretion is brought about by norepinephrine secreted by the postganglionic sympathetic nerves that innervate the pineal gland. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. 2012. Chapter 14. Electrical Activity of the Brain, Sleep--Wake States, Circadian Rhythms. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks Eds, Ganongs Review of Medical Physiology, 24e. 