Fig. 1.

Components of the mammalian circadian clock system. Changes in light due to the day/night cycle are directly detected by eyes. The light information is transported to the suprachiasmatic nucleus (SCN) in the anterior hypothalamus by the retinohypothalamic tract (RHT). The SCN functions as a master circadian oscillator where circadian rhythmicity is generated. The generated circadian rhythmicity is converted into output pathways that control the behavior, physiology and metabolism of the organisms. These environmental signals, core circadian oscillator and output rhythms are three basic components of circadian clock system.