Obstructive sleep apnea is a risk factor for diabetes and cardiovascular disease, by unknown mechanisms. The investigators hypothesize that sleep apnea changes glucose and lipid metabolism during sleep, which over time could lead to diabetes and cardiovascular disease. This study examines metabolic changes during sleep in patients with obstructive sleep apnea. Patients accustomed to CPAP therapy are enrolled to undergo sleep studies, either on CPAP therapy or after withdrawing from CPAP for 3 nights. During sleep, blood samples are obtained so that metabolic function can be compared between sleep apnea and CPAP nights.