Abstract:
Many organisms show rhythmicity in physiological processes, metabolism, behaviours
and reproduction. Depending upon the period, rhythms are classified as - ultradian rhythms
(milliseconds to hr), circadian rhythms (24 hr), tidal rhythms, annual rhythms etc. The
rotation of the earth on its axis, its revolution around the sun and the revolution of the moon
around the earth subject organisms to geophysical cycles such as daily cycles (24 hr) of light,
temperature and to seasonal changes. Organisms appear either to cope with the daily changes
in the external environment or even to make use of these periodic changes. Many organisms
from simple unicellular beings to complex mammals exhibit daily rhythms (DeCoursey,
2004) and this 24 hr rhythmic pattern is not merely a response to the external environment
and is in fact produced endogenously as was first demonstrated by Jacques de Mairan, in 1729
in the heliotrope plant (de Mairan, 1729). These daily rhythms are characterised by several
features: 1) they are endogenously generated (produced within the organism) self-sustaining
(continue to oscillate in the absence of any external time cues) with a free running periodicity
of ~ 24 hr, hence the name circadian (from Latin ‘circa’ – approximately and ‘diem’ – a day)
2) they are entrainable (can be synchronized to periodic external conditions like light,
temperature etc) and 3) they are temperature compensated (circadian period is not
significantly altered by physiologically tolerable changes in temperature). The circadian
system can be conceptualised as consisting of input pathways, which synchronize the clock to
the environment; central oscillator, which maintains time; and output pathways, which convey
information from central oscillator to temporally organize physiology and behaviour.
