Abstract
Knowledge on mechanisms underlying the mammalian circadian system, on the effect of light and season of the year on the system and on the role of melatonin improves the understanding of daily rhythms or of chronobiological disorders and leads to new therapies. Many biochemical, physiological, and behavioral processes in mammals including humans exhibit circadian rhythms. Among them are endocrine rhythms in melatonin, adrenocorticotropic hormone (ACTH), corticosteroids, growth hormone, and prolactin as well as rhythms in the rest-activity and sleep–wake cycle, in body temperature, and many other variables. The rhythms are controlled by a circadian pacemaker located in the suprachiasmatic nuclei (SCN) of the hypothalamus. In a non-periodic environment, the pacemaker free-runs with a period close to, but not equal to 24 hour. To the 24-hour day, the phase and period of the circadian pacemaker are entrained mainly by alternation of environmental light and darkness—namely, by the light part of the day. Classically, photic stimuli in the evening and early night phase delay, whereas in the late night and morning phase advance the circadian pacemaker; light expo-sure during the day does not usually affect the pacemaker's phase. Photic information is conveyed to the SCN by way of a direct retinal projection, the retinohypothalamic tract, and by other pathways—namely, by the geniculohypothalamic tract.
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