Circadian Rhythm – A Review

Abstract

The physiological processes of organisms are regulated by a circadian rhythm. The circadian rhythm is regulated by the wavelength, intensity, timing and duration of the light stimulus. Biological rhythms affect the sleep–wake cycle, migration behaviour in birds, seasonal fattening, hibernation and reproductive cycles in animals. The circadian rhythm is ubiquitous in nature. Circadian rhythms appear to be generated at the cellular level. Daily biological rhythms are endogenously controlled by self-contained circadian clocks. The period length is controlled by a circadian oscillator (clock). 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. Adverse effects may ensue when the sleep-wake cycle is out of phase with the rhythms that are controlled by the circadian clock. Circadian rhythm is related to the light/dark cycle of the solar day but it also persists in constant conditions. The environmental cues entraining the circadian rhythm are called Zeitgebers or circadian synchronizers. Temporal restrictions of feeding (RF) can phase-shift behavioural and physiological circadian rhythms in mammals. Phase advances of circadian rhythms happens for instance, in the liver, kidney, heart, pancreas and some brain structures, uncoupling them from the control of the SCN, whose entrainment to light remains intact.