The pathophysiological links between obstructive sleep apnea syndrome (OSAS) and cardiovascular mortality are incompletely understood. We aimed to contribute to a better characterization by using comprehensive biomarker profiling quantifying hemodynamic cardiac stress, cardiomyocyte injury, inflammation, endothelial function, matrix turnover and metabolism. In 65 patients with moderate or severe OSAS apnea–hypopnea index (AHI) 39±20/h and 33 patients with no or mild OSAS (AHI 8+4/h), B-type natriuretic peptide (BNP), N-terminal-pro-BNP (NT-proBNP), high-sensitivity cardiac troponin I (hs-cTnI), interleukin-6 (IL-6), vascular endothelial growth factor (VEGF), matrix metalloproteinase-9 (MMP-9), and insulin were measured before and after sleep. In a subgroup measurements were repeated in a second night with continuous positive airway pressure (CPAP). Patients with moderate/severe OSAS had higher insulin before sleep median (interquartile range), 36.4 (21.9–52.1) vs. 20.8 (10.6–32.8)mU/mL; p=0.006, higher IL-6 after sleep 1.00 (0.73–1.58) vs. 0.72 (0.48–0.94)pg/mL; p=0.005, and larger relative overnight reduction in BNP −9 (−35–0) vs. −3 (−21–13)%; p=0.04 than those with mild/no OSAS. Insulin before sleep was the only independent predictor of moderate/severe OSAS. Insulin before and IL-6 after sleep were independent predictors of severe OSAS, and when combined provided high diagnostic accuracy for severe OSAS (area under the receiver operator characteristic curve 0.80; 95%-confidence interval 0.69–0.91). In contrast, there were no significant differences in NT-proBNP, hs-cTnI, VEGF, and MMP-9 between moderate/severe and mild/no OSAS. Short-term CPAP had no impact on biomarker concentrations before and after sleep. Significant OSAS is characterized by a distinct biomarker profile including high insulin before and high IL-6 after sleep. Display omitted •We measured a panel of biomarkers in patients with and without significant OSAS.•We found a relevant diurnal variation of a number of biomarkers in OSAS patients.•The association between biomarkers and OSAS depended on the time of measurement.•Insulin measured before sleep was an independent predictor of severe OSAS.•Interleukin-6 measured after sleep was an independent predictor of severe OSAS.