Background: Obstructive sleep apnea syndrome (OSAS) is one of the representative sleep disorders believed to be associated with metabolic syndrome. Nasal continuous positive airway pressure (nCPAP) ventilation is the first choice therapy for OSAS, which has been reported to cause an improvement in body fat mass, hepatocellular damage and hypertension. Study Objectives: We evaluated whether the changes in the body weight observed in patients with OSAS may have potential associations with the sleep time during which the patients are under nCPAP. (...) Method: A total of 194 patients (148 obese and 46 non-obese) in whom nCPAP use was present for more than 70% of the nights were enrolled in this study. Using the electronic records of the night use time for CPAP devices, we examined whether the habitual sleep time during nCPAP is associated with changes in body weight. Results: In the non-obese OSAS group, the patients with night time use of nCPAP devices for 6 to 7 hours showed the greatest and the most sustained decrease in body weight. In the obese patients with OSAS, on the other hand, a U-shaped relationship has been demonstrated between a percent weight gain over 9 years and a minimal weight gain in patients with habitual sleep for 5 to 6 hours. Conclusions: These results suggest that changes in body weight in patients with OSAS are associated with habitual sleep time. eww141010dxn. (shrink)