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Cell physiology is regulated along the 24-h time-scale by a circadian timing system composed of molecular clocks within each cell and a central coordination system in the mammalian molecular clock is made of interconnected molecular loops involving at least 12 circadian genes. The cellular clocks are coordinated by the suprachiasmatic nucleus (SCN) and a hypothalamic pacemaker, which also helps the organism to adjust to environmental cycles. The biological functions of most living organisms are organized along an approximate 24-h time cycle or circadian rhythm. The endogenicity of the circadian rhythms has been demonstrated in microorganisms, plants, and all kinds of animal species including man. These endogenous rhythms govern daily events such as sleep, activity, hormonal secretion, cellular proliferation, and metabolism.