author(s)
Sohan Vir Singh and Sunil Kumar

abstract
Circadian rhythm regulates the physiological processes of animals. The ambient conditions viz. wavelength, intensity of light, timing and duration of the light stimulus, temperature etc deregulates the circadian rhythm. Circadian rhythms are generated at the cellular level in living organisms. Sleep–wake cycle, fattening, hibernation, reproductive behavior and migration pattern/ behavior in birds are controlled by biological rhythms in animals. Biological rhythms are controlled endogenously by self-contained circadian clocks. The timing of sleep and wakefulness under natural conditions is in synchrony with the circadian control of the sleep cycle and all other circadian-controlled rhythms. Generally, the circadian rhythm is regulated by the light/dark cycle of the solar day, but it also persists during constant conditions. Change in the photoperiod (daylength) is the most important environmental factor for the deviation in physiology and behavior, timing of migration, hibernation, reproduction etc. Arctic animals (ptarmigan, reindeer) show circadian rhythms only in the parts of the year that have daily sunrises and sunsets. Circadian clocks have been reported in each and every mammalian cell including cardiovascular system, such as cardiomyocytes and vascular smooth muscle cells. Circadian rhythmicity of physiological systems can have several purposes in reproduction. The important functions are viz. coordination of ovulation, receptivity and wakefulness in the female with activity/wakefulness in the male. The advantages to a species of restricting onset of fertility and the resulting pregnancy and offspring care to favorable times of the year are obvious.