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The three components of the circadian system: (1) input pathways such as light and exogenous melatonin, which provide timing signals to the central circadian clock, (2) the central clock, suprachiasmatic nuclei (SCN) in the hypothalamus which generates the rhythms, and (3) the output rhythms which includes endogenous melatonin and molecular peripheral circadian clocks contained in most tissues. Many of these output rhythms can feedback to the central circadian clock.
