Abstract:
Organisms ranging from bacteria to mammals exhibit daily rhythms in various
behavioural and physiological processes. These 24-hr rhythms are not mere passive
responses to environmental cycles because they have been found to persist even under
constant laboratory conditions, with near 24-hr period (from „circa‟ - about, „dies‟ - a day;
hence circadian), which suggests the presence of endogenous rhythm generating systems
(Dunlap et al., 2004). Endogenous, near 24-hr rhythms match their period and maintain a
stable phase-relationship with daily environmental cycles through a process known as
„entrainment‟, using environmental time-cues (zeitgebers) such as light, temperature,
humidity, food availability, and social interaction cycles (Dunlap et al., 2004; Sharma and
Chandrashekaran, 2005). The period of these rhythms is largely protected from changes in
ambient conditions such as temperature, nutrition and pH, within physiological range, thus
giving it a compensatory ability that allows organisms to maintain their rhythmic functions
stably in the face of environmental fluctuations. It is thus believed that the ability of
circadian timing systems to achieve temporal organization in behaviour and physiology has
evolved as an adaptation to daily environmental changes arising due to the rotation of earth
about its own axis (Pittendrigh, 1993; Sharma, 2003a; Dunlap et al., 2004). Further, it is
believed that circadian clocks confer adaptive benefit to living organisms by timing their
behavioural and physiological processes to appropriate time of the day so as to maintain
coordination between internal rhythms and cyclic external environment (Pittendrigh, 1993;
Sharma and Joshi, 2002; Sharma, 2003a; Dunlap et al., 2004).
