PER1 and sleep disorder: Although the direct role of PER3 in the etiology of mood and sleep disorders is not yet known, it is thought that post-transcriptional modifications and ligand interactions involving PER3 interfere with the phosphorylation of PER1 and PER2—mediating the stability and turnover of these core circadian clock proteins and altering the period length and coordination of circadian rhythms [30,41,42].