Answer: melatonin

AnswerCrossword Clue
MELATONINBiorhythm-regulating hormone
melatonina hormone secreted by the pineal gland during the hours of darkness
melatoninhormone secreted by the pineal gland in inverse proportion to the amount of light received by the retina, important in the regulation of biorhythms: in amphibians, it causes a lightening of the skin