Abstract:
Most living organisms experience predictable variation in environmental factors due to various geophysical cycles present on the earth. Cyclic variation in the environment presents opportunities and risks to organisms at various times of the day, month or year (Enright, 1970; Pittendrigh, 1981a; Daan, 1981). Internal time-keeping mechanisms are thought to have evolved in organisms to exploit the temporal order in the environment so as to increase reproductive fitness (Moore-Ede and Sulzman, 1981; Sharma, 2003; Vaze and Sharma,
2013). Among geophysical cycles present in the environment, the daily cycle (of period ~ 24 hours) due to the rotation of the earth about its axis is probably the most consistent cycle experienced by organisms (Moore-Ede, 1986). Circadian clocks (with periodicity close to 24 hours) are ubiquitously seen in most eukaryotic and some prokaryotic organisms. These internal clocks may have evolved as a consequence of daily selection pressures on preferentially allocating different physiological tasks to different parts of the day (Moore-Ede and Sulzman, 1981).
