Purpose The objective of this study is to investigate the effects of laparoscopic sleeve gastrectomy (LSG) on the polysomnographic parameters related to OSAS. Materials and Methods We conducted this 3-year prospective cohort study in a tertiary care center between December 2016 and December 2019. In total, we enrolled 31 patients with severe obesity who underwent full-night polysomnography (PSG) before LSG. Later, the patients were re-evaluated by full-night PSG 12 months after the surgery. Results The mean age of the patients was 44.1 +/- 9.6 years. The mean body mass index (BMI) decreased significantly from a mean value of 49.8 +/- 8.5 kg/m(2) at baseline to 33.2 +/- 8.2 kg/m(2) and a percent BMI (%BMI) reduction of 33.8 +/- 10.4% and a percent total weight loss (%TWL) of 35.4 +/- 10.8% was achieved on the same day of the postsurgical PSG (p < 0.001). There was a remarkable improvement in the AHI (baseline: 36.1 +/- 27.1, 12 months after the surgery: 10.3 +/- 11.8; difference: 25.8 +/- 22.8 events per hour) (p < 0.001). Importantly, there was a decrease in the percentage of non-rapid eye movement (NREM) 2 (p < 0.001), whereas NREM 3 and REM stages witnessed a significant increase (p = 0.001 and p < 0.001, respectively) after the surgery. Conclusion The results of this study showed that weight loss after LSG yields improvement not only in AHI but also in many polysomnographic parameters such as sleep quality and desaturation indices.