In this model, the circadian molecular clock acts as a cell-autonomous rhythm generator (center) that produces rhythmic patterns of gene expression. At the organism level, the system has two parts: a central pacemaker housed in the central nervous system within the suprachiasmatic nucleus (SCN) of the hypothalamus and local pacemakers within peripheral tissues responsible for physiological outputs. To set the biological time of day (the circadian phase), the clock within the central pacemaker converts rhythms in external light into synchronized oscillations in hormone secretion (melatonin and hypothalamic pituitary axis), autonomic neural activity, arousal, appetite, and core body temperature. These rhythmic internal cues are converted by oscillators within peripheral cells into tissue-specific circadian rhythms. Circadian rhythms can modify the external environment around cells and, in so doing, can affect the amplitude and phase of the peripheral clock through feedback regulation.