Figure 1.

The circadian clock. The core circadian clock consists of a molecular transcriptional-translational feedback loop in which the transcription factors, CLOCK and BMAL1, heterodimerise and induce expression of the core clock genes, Per and Cry, via E-box response elements. PER and CRY then feedback onto CLOCK and BMAL1 by inhibiting their transcriptional activity. This feedback loop cycles with a period of around 24 h, therefore it must be continually readjusted to be aligned with the external world. The primary time cue for this is the daily light/dark cycle. Light information is transmitted via the retinohypothalamic tract (RHT) directly to the master clock in the suprachiasmatic nucleus (SCN).