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.