Circadian rhythms are cycles in physiological, behavioural,
psychological and other domains that recur approximately
every 24 h. Such rhythms are driven by an endogenous
circadian timekeeping system, whose molecular basis
consists of the interlocking transcription and translation of
a panel of "clock" genes, the expression of which then
regulates gene expression and cellular function in a tissuespecific
manner. Such rhythmic expression of clock genes
is observed in most tissues, including the master circadian
pacemaker in the suprachiasmatic nucleus of the hypothalamus,
other CNS sites and peripheral organs. Therefore,
the circadian system exerts considerable control over
physiology and behaviour, and equally dysfunction of this
circadian system may result in detrimental pathophysiological
and psychopathological consequences