The circadian clock is an intrinsic time-keeping mechanism that controls the daily rhythms of numerous physiological processes, such as sleep/wake behavior, body temperature, hormone secretion, and metabolism. Circadian rhythms are generated in a cellautonomous manner through transcriptional regulatory networks of clock genes. In the core feedback loop, the transcription factors CLOCK and BMAL1 activate expression of Period (Per1 and Per2) and Cryptochrome (Cry1 and Cry2) genes.[1] Cryptochromes (CRYs) are structurally related to UV/blue-sensitive DNA repair enzymes called photolyases but lack their ability to repair pyrimidine dimers generated by UV exposure.[2]