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Circadian rhythms are autonomous, systemic cycles of behavior and physiology that synchronize organismal function at the cell and tissue level in anticipation of the 24-h rotation of the Earth. A common feature of modern “24/7” life is the routine disruption in these evolutionarily conserved endogenous circadian cycles, due to the rise in shift work, jet travel across time zones, exposure to blue light–emitting devices at night, and disrupted sleep. A transformation in understanding the molecular basis of circadian disorders has generated a new wave of research on metabolic disease, inflammation, aging, and cancer. This chapter provides an overview of (1) the basic biology of the circadian system; (2) primary circadian rhythm and interrelated sleep disorders; and (3) the role of the circadian system in both normal human physiology and disease states. Lastly, we review the rapidly emerging field of chronobiology as a pathway for novel diagnostic and therapeutic activity. A glossary of terms used in circadian biology is summarized in Table 475-1.

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TABLE 475-1Glossary of Terms Used in Discussion of the Circadian System

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