Abstract:
Life forms on the earth have to experience and cope with several diurnal and annual
geophysical cycles such as light and temperature. It is believed that organisms have evolved
mechanisms in order to accomplish the task of synchronizing their behavioral, physiological and
biochemical processes to the periodic environmental cycles (Vaze & Sharma, 2013), through the
evolution of biological timekeeping systems or clocks. Many of the rhythmic phenomena in a
diverse range of organisms have been shown to be regulated endogenously by their clocks
(Pittendrigh, 1960), rather than just a mere response to the periodic variations in the
environmental conditions. Biological oscillators with a periodicity of close to 24 hours (circa =
almost, dian = day) are referred to as circadian clocks. In addition, these oscillators need to need
to satisfy a few criteria in order to be called circadian clocks, such as,
1) they must be endogenously generated
2) they must be self-sustained – i.e., the oscillation must persist (free-run) even in the
absence of any periodic time-cue (or, zeitgeber; zeit = time, geber = giver)
3) they can be synchronized by periodic light/dark cycle by a process known as
‘entrainment’
4) they must be temperature-compensated - the period of the oscillator should remain
unaltered within a physiologically tolerable range of temperatures (Pittendrigh, 1960).
