: Diurnal, nocturnal or seasonal modes of behavior are not passive responses to changes in the environment; rather, they are generated by an endogenous circadian pacemaker, entrained by a few environmental cues like lightdark cycles. Circadian clock mechanisms involve periodic gene expression, synchronized by a hierarchically superior structure located in mammals in the hypothalamic suprachiasmatic nuclei. Cycles of sleep and wakefulness are the most conspicuous circadian rhythm. Since modern humans use artificial light to extend their period of wakefulness and activity into the evening hours, they adhere to a shortnight sleep schedule with a highly consolidated and efficient sleep. As shown by studies in artificial long nights, modern humans may be sleepdeprived. Humans have also increasingly insulated themselves from the natural cycles of light and darkness. Still, the human circadian pacemaker has conserved a capacity to detect seasonal changes in day length. A mood disorder involving a recurring autumn or winter depression (seasonal affective disorder, SAD) is related to latitude, with the number of cases increasing with distance from the equator. SAD is ameliorated by using brilliant light. In nonseasonal depression, mood typically fluctuates daily, with improvement over the course of the day, and various physiological functions exhibit an altered circadian pattern, suggesting a link with circadian disruption. Treatment of circadian rhythm disorders, whether precipitated by intrinsic factors (e.g., sleep disorders, blindness, mental disorders, aging) or by extrinsic factors (e.g., jet lag, shift work) has led to the development of a new type of agents called "chronobiotics," among which melatonin is the prototype.