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Circadian (from the Latin word circa, meaning 'about', and dies, meaning 'day') rhythms are the rhythms occurring within a period of 24 hours and are endogenously driven in biochemical, physiological or behavioral processes [1]. Circadian rhythms are also entrained by the external environmental cues called zeitgebers, the primary one of which is day light [2]. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria [3]. The study of the biological processes that vary during different time of the day, is called chronobiology. A number of studies on mammalian species have confirmed that the circadian pacemaker resides in the supra-chiasmatic nucleus (SCN) that is a part of hypothalamus in the brain [4]. The SCN receives signals from the environment and provides the principal timing cues for synchronizing the daily oscillations of different proteins in brain tissues [5]. So, different physiological activities and behaviors of human and animals vary during 24 hours.

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