Recent discoveries have highlighted how the internal coincidence of the circadian clock can change phases to synchronize with external environmental cycles[44], as has been shown by mutations of PER2 and CSN1KD in familial advanced sleep phase syndrome[14,15], mutations or SNPs of ARNTL, DEC1 and RORB in bipolar disorder[45], and mutations of CRY1 in familial delayed sleep phase disorder[12]. This evidence concerns the gene PER2 and bipolar disorder.