OBJECTIVE The aim of the study was to investigate the impact of obstructive sleep apnea (OSA) on the QT interval variability and duration in patients during different sleep stages. METHODS Polysomnographic recordings of 28 (13 male, 15 female) patients with OSA and 30 (15 male, 15 female) patients without OSA were analyzed. The QT interval variability index (QTVI) and the corrected QT interval (QTc) analyses were performed using two awake, 3-4 non-rapid eye movement (NREM) and three rapid eye movement (REM) sleep episodes (each 300 s). The Bazett formula, linear, and parabolic heart rate correction formulas with two separate α values were used. RESULTS QTVI was statistically higher in OSA than in non-OSA patients for males while awake (awake -0.7 ± 0.3 vs -1.2 ± 0.2, p = 0.001; NREM ‒0.9 ± 0.4 vs -1.1 ± 0.3, p = 0.110; REM ‒1.1 ± 0.3 vs -1.3 ± 0.2, p = 0.667) and for females in all wake-sleep stages (awake -0.3 ± 0.7 vs -0.9 ± 0.5, p = 0.001; NREM ‒0.3 ± 0.5 vs -0.8 ± 0.4, p = 0.002; REM -0.3 ± 0.5 vs -1.0 ± 0.4, p < 0.001). QTVI was significantly higher during awake compared to sleep stages in OSA males (p < 0.05); no difference between wake-sleep stages was found in females (p > 0.05). Significant gender differences in QTVI existed in OSA patients during sleep (p < 0.05) but not while awake. No significant differences in QTc between patients groups were observed. CONCLUSIONS OSA is associated with increased QT variability. REM sleep per se does not increase QTVI. In OSA patients, QTVI might be a more useful measure to detect ventricular repolarization abnormality than measures of QTc.