Figure 1.

General overview of the functional organization of the circadian system in mammals. Inputs: environmental periodical cues can reset the phase of the central pacemaker so that the period and phase of circadian rhythms coincide with the timing of the external cues; Central pacemakers: the suprachiasmatic nuclei (SCN) is considered to be the major pacemaker of the circadian system, driving circadian rhythmicity in other brain areas and peripheral tissues by sending them neural and humoral signals (such as melatonin, secreted by the pineal gland (P)). The SCN receives light-dark cycle information through the retinohypothalamic tract (RHT). Peripheral oscillators: most peripheral tissues and organs contain circadian oscillators. Usually, they are under the control of the SCN; however, under some circumstances (e.g., restricted feeding, jet lag and shift work), they can desynchronize from the SCN; Outputs: central pacemakers and peripheral oscillators are responsible for the daily rhythmicity observed in most physiological and behavioral functions. Some of these overt rhythms (physical exercise, core temperature, sleep-wake cycle and feeding time), in turn, provide feedback, which can modify the function of the SCN and peripheral oscillators, (redrawn from [11]).