FNDC5 and obstructive sleep apnea syndrome: Serum irisin level was higher in quite severe OSA group (345.74 ± 99.53 ng/ml) than that in control group (140.29 ± 97.72 ng/ml) and that in nonsevere group (201.13 ± 123.91 ng/ml) (control: p ≤ 0.001; nonsevere: p ≤ 0.001), and it was also higher in severe group (287.17 ± 143.52 ng/ml) than that in control group (p < 0.01).