ABSTRACT
In humans, endocrine secretions and metabolic function display clear 24-hour rhythms that are mainly controlled by sleep-wake homeostasis and by oscillatory signals generated by the suprachiasmatic nucleus (SCN) of the hypothalamus. The circadian pacemaker in the SCN is primarily entrained by the light-dark cycle. In diurnal species, such as the human, neuronal activity in the SCN is maximal during light exposure and sleep occurs during the dark phase, when SCN neuronal activity is low.
