Objective: The main objective of this study was to investigate the quality of sleep in patients with obstructive sleep apnea who have type 2 diabetes mellitus and to analyze the findings of overnight polysomnography. Participants and Methods: A cross-sectional retrospective study was conducted involving 80 participants (40 in the research group and 40 in the control group) aged 26 to 78 years. All participants included in the study were processed at the Center for Sleep Medicine of the Clinical Hospital Center Split during the period from 2013 to 2021. Patients completed the PSQI, ESS, and STOP questionnaires and underwent diagnostic overnight polysomnography. Individuals recorded on a device for overnight polygraphy, those younger than 18 years, and those with insufficient data were excluded from the study. The control group was also formed from the digital database of the Center for Sleep Medicine and was matched with the research group by the level of OSA (AHI 5-14,9=mild, 15-29,9=moderate, ≥30=severe), age, gender, and BMI. Results: The risk for developing OSA measured by the STOP questionnaire is significantly higher in the study group compared to the control group (38 (95%) vs. 32 (80%), P=0.044). The assessment of excessive daytime sleepiness measured by the ESS scale (P=0.885) and the subjective assessment of sleep quality and sleep hygiene parameters measured by the PSQI questionnaire (P=0.147) did not significantly differ between the groups. The wakefulness period after sleep onset obtained by overnight polysomnography is significantly higher in the study group compared to the control group (79.6 (55.8-104) vs. 62.7 (36-83.6), P=0.028), while sleep time, duration, and latency did not significantly differ. There was also no significant difference in sleep architecture and the frequency of microarousals. Conclusion: This study showed that patients with type 2 diabetes have a higher risk of developing OSA as measured by the STOP questionnaire and a longer wakefulness period after sleep onset compared to those with OSA without diabetes. The study did not find a significant difference in the subjective assessment of sleep quality measured by the PSQI questionnaire and polysomnographic parameters of sleep duration, latency, and architecture between the study and control groups.