Abstract:
Almost all living organisms on the earth perceive robust 24-hr cycles of abiotic variables
such as light, humidity and temperature, which occur as an inevitable consequence of
unceasing rotation of the earth around its axis. Unicellular organisms such as bacteria to
complex living systems such as human beings exhibit 24-hr rhythms in various behavioral
and physiological processes. These rhythms persist under constant laboratory conditions with
near 24-hr periodicity (hence circadian; circa - about, dies - day), which indicates that daily
rhythms are not simply passive responses to 24-hr environmental cycles, but are the
expression of some endogenous rhythm-generating systems (Dunlap et al., 2004).
Scheduling of biological functions at specific time of the day is believed to be the primary
function of these endogenous oscillators (Roenneberg et al., 2003a), which they achieve by
using various environmental time cues such as light, temperature, social cues through a
process known as entrainment (Johnson et al., 2003). The system comprising of core
endogenous oscillators, mechanisms to sense environmental time cues (zeitgebers) and
transduction mechanisms by which the oscillators regulate circadian rhythms are collectively
known as “circadian clocks”.
