CRY1 and obstructive sleep apnea syndrome: In severe OSA, the expression of CRY1 and PER3 (r = 0.746, p < 0.001) at midnight was highly correlated (Figure 4A) but the correlation between CRY1 and AHI (r = 0.309, p < 0.001) and between PER3 and AHI (r = 0.378, p < 0.001) was not very high.