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Life existence on earth is based on day/ night time cycle. Circadian rhythms (CR) are essential endogenous autonomous oscillators of physiological activities resulting from 24- hours day/night cycles, which allows organisms to adapt to the fluctuating environment. The pivotal pacemaker of the CR is the Suprachiasmatic Nucleus (SCN) which plays pivotal roles in regulating peripheral tissue clocks enhancing metabolic activities and practically every physiological activities in the body of living organisms through the release of endogenous regulatory substances. An unaltered circadian rhythm is a way to good health, programming the array of physiological activities in the body in a chronological/orderly manner. This review appraises the operation of the biological clock as it equips the body of organisms to perform daily routines in a healthy way under the influence of external cues such as food, arousal stimuli and light. The efficiency of many biochemical processes depends on a healthy circadian rhythm.
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