Participants with a history of moderate severe sleep apnea (AHI>20) will be enrolled if they meet inclusion/exclusion criteria. They will report to the sleep laboratory on two nights, (1) after continuing their CPAP or (2) after discontinuing CPAP for 3 nights. The order of observation will be randomized. A standard research dinner is provided before each study. IV's are placed so that blood can be sampled at frequent (20 min) intervals throughout the night. In the morning after each study, a glucose tolerance test and endothelial function study (endoPAT) are performed. Serum samples will be analyzed for glucose, insulin, free fatty acids, triglycerides, hormones, and other metabolic biomarkers.