Objectives: Obstructive sleep apnea (OSA) is a disorder characterized by repetitive cessations of breathing during sleep. One of the most prominent symptoms of OSA is excessive daytime sleepiness, associated with micro arousals from sleep which are caused by respiratory events. Another feature of the OSA linked to respiratory related arousals might be impaired sleep architecture resulting in decreased subjective sleep quality. The main aim of this research was to assess and compare the subjective sleep quality and daytime sleepiness and objective whole-night polysomnography findings in OSA patients. Patients and methods: A total of 317 adult OSA patients from the Split Sleep Medicine Center, with an age range from 18 to 82 years, were included in this study. All the patients filled in the questionnaires: Epworth sleepiness scale (ESS), assessing daytime sleepiness, and Pittsburgh Sleep Quality Index (PSQI), assessing subjective sleep quality. Following completion of the questionnaires, the patients underwent whole-night polysomnography. Results: Sleep architecture changed with OSA severity, with an increased amount of sleep stage N2 (66.41 ± 11.93% in mild OSA, 69.12 ± 11.43% in moderate OSA and 73.45 ± 14.74% of sleep stage N2 in severe OSA, F = 8.635, P < 0.001) and decreased amount of deep sleep stage N3 in more severe forms of OSA (11.67 ± 8.89 % in mild OSA, 11.10 ± 7.35% in moderate OSA and 6.88 ± 7.63 in severe OSA, F = 12.701, P < 0.001). There was also a correlation between OSA severity and excessive daytime sleepiness assessed by ESS (r = 0.25, P < 0.001). Patients who had poorer sleep efficiency had worse subjective sleep quality assessed by PSQI (r = -0.21, P = 0.001). There were no statistically significant differences in sleep architecture among patients with good and bad sleep quality according to PSQI (P > 0.05), and among those with normal and excessive daytime sleepiness, according to ESS (P > 0.05). Conclusions: The obtained results demonstrated that OSA severity was associated with an impairment of the sleep architecture and with more pronounced excessive daytime sleepiness. Sleep efficiency, but not the sleep architecture of OSA patients correlated with subjective sleep quality assessed by PSQI. However, no differences in sleep architecture among OSA patients with and without excessive daytime sleepiness assessed by ESS were found.