Melatonin

Daily cycles in physiology and behavior appear to be a universal feature of living organisms. An intrinsic body clock which resides in the brain’s suprachiasmatic nucleus (SCN) regulates a complex series of physiological and behavioral rhythms, including sleep and wakefulness. The endogenous clock naturally runs at a somewhat slower rhythm than the solar 24-hour cycle but is normally entrained by the light-dark cycle to match the environmental rhythm. Although a master clock resides in the brain SCN, a functional clock appears to reside in most cells of the body. In all these tissues, at least some output genes are controlled at the transcriptional level directly by clock proteins; others appear to be regulated by cascades of circadian transcription factors or neuronal stimuli.1.