PER1 and lip and oral cavity carcinoma: The key mechanism may be that chemotherapy causes circadian rhythm disruption by affecting the expression of core clock genes (such as BMAL1, Clock, PER, and CRY) and the levels of hormones (such as cortisol and melatonin) [53], which leads to daytime sleepiness, fatigue, and an increased risk of OSA in patients with oral cancer.