A selective evolutionary advantage seems to be conferred on individuals whose physiological rhythms are synchronized to environmental conditions. Thus, the majority of organisms studied show twenty-four hour rhythms in physiological processes termed circadian rhythms. Disturbances in circadian rhythms are known to contribute to a variety of diseases and to impair mental and physical performance. The circadian system consists of three conceptual components, a timekeeping oscillator, input pathways providing environmental information, and output pathways sending timing information to peripheral clocks and organs. The timekeeping oscillator or clock resides, in mammals, in the suprachiasmatic nucleus (SCN) a bilateral structure located in the hypothalamus. This circadian clock is synchronized (entrained) to the environmental light-dark cycle via a direct axonal projection (retinohypothalamic tract, RHT) from a specialized subset of retinal ganglion cells. SCN neurons ouput timing information via projections to other hypothalamic nuclei and by releasing neurohumoral factors.