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 . Circadian rhythms are also entrained by the external environmental cues called zeitgebers, the primary one of which is day light . Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria . 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 . The SCN receives signals from the environment and provides the principal timing cues for synchronizing the daily oscillations of different proteins in brain tissues . So, different physiological activities and behaviors of human and animals vary during 24 hours.
Rafiq, Muhammad; Mahmood, Zahid; and Ali, Sajed
"Circadian variation of brain proteins: Importance in experiments,"
Pakistan Journal of Neurological Sciences (PJNS): Vol. 13
, Article 11.
Available at: https://ecommons.aku.edu/pjns/vol13/iss2/11