PER3 (period circadian regulator 3) is a core component of the circadian clock that generates approximately 24-hour rhythms in gene expression to regulate physiological processes including metabolism, sleep, body temperature, and immune function 1. As a transcriptional repressor, PER3 forms the negative limb of the circadian feedback loop, interacting with CLOCK-BMAL1 heterodimers to inhibit their activity 2. Unlike PER1 and PER2, PER3 has a redundant role in circadian rhythm maintenance but plays a critical role in sleep-wake timing and sleep homeostasis through transcriptional regulation of sleep-related genes 3. A variable number tandem repeat (VNTR) polymorphism in PER3 significantly associates with chr1 and sleep phenotypes: the 4-repeat allele correlates with earlier sleep timing and morning preference, while the 5-repeat allele associates with delayed sleep phase 14. PER3 variants also influence responses to sleep loss, light sensitivity, and cognitive performance during circadian misalignment 3. Disease relevance includes associations with delayed sleep phase syndrome, where 75% of patients were homozygous for the shorter allele 1, fibromyalgia syndrome susceptibility 5, and seasonal depression via altered glucocorticoid signaling 6. PER3 dysfunction contributes to circadian rhythm disruption linked to cardiovascular disease, cancer, and metabolic syndromes 2.