Circadian rhythms respond primarily to light and dark and affect most living things. These are more synchronized in birds than mammals. Circadian clock functions as a timing reference allow organisms fluctuations in their environments and are the basis for the transduction of seasonality from photoperiod. Organisms maximize their fitness by synchronizing their physiology and behavior with the abiotic and biotic features of their environments. This can be possible by endogenous circadian clocks, when they prevailing with light, food and socio environments, determine temporal patterns of events during the day in daily functions as activity rest, sleep wake and feeding patterns. At the regulatory level, circadian clocks govern changes in physiology and behavior within each day; hence, behavioral and physiological functions exhibit daily and circadian rhythms under periodic daily Light Dark (LD) cycles and constant dim light or darkness, respectively.