ABSTRACT
The rotation of the earth about its axis relative to Sun causes the formation of day/night cycles with a regular periodicity. Accordingly, all living organisms adjust to this external clock by choosing their respective environmental niches. They do so by expressing endogenous rhythms that can measure the passage of time on an approximate 24 h scale. Hence these rhythms are known as circadian (circa= approximately; dies= a day) rhythms. The underlying machinery responsible for the circadian rhythms is known as circadian clock or biological clock that has three important components, i.e.
