In the OSA group, on the gene expression level, all analyzed circadian clock genes correlated at a given time point (evening/morning) with HIF-1α and HIF-1β gene expression; the only exception was a lack of association between PER1 and HIF-1α in the morning (R = 0.050, p = 0.73). The gene discussed is CLOCK; the disease is obstructive sleep apnea syndrome.