The investigators' overall goal is to compare the effect of CPAP treatment on intermediate cardiovascular risk measures in obese versus lean patients with obstructive sleep apnea (OSA). The overall hypothesis is that, adjusting for OSA severity and obtaining normative data from non-OSA subjects with comparable amounts of visceral adiposity, the two OSA groups will have comparable improvements in daytime sleepiness, but that the cardiovascular and metabolic improvements following CPAP therapy will be decreased in OSA patients with increased visceral adipose tissue. The investigators anticipate that, although there will be a greater absolute change in markers of sympathetic activity, inflammation and oxidative stress in obese compared to lean OSA patients following CPAP treatment, the levels will still be abnormally high in the obese patients resulting in the decreased improvements in insulin resistance, arterial blood pressure, and vascular health in obese versus lean OSA patients.