"Reversibility of Cardiovascular Injury With CPAP Use: Mechanisms Involved"

Inclusion Criteria: * subjects naïve of CPAP therapy just recently diagnosed with moderate to severe obstructive sleep apnea (OSA). The degree of OSA is defined by results of a comprehensive polysomnogram requiring an apnea hypopnea index (AHI) \>15 events/hour and Epworth score \>10, or an AHI \>20 events/hour * either sex * any race * between 21-50 years old. Exclusion Criteria: * Systolic blood pressure \>160 mmHg, or diastolic blood pressure \>100 mmHg * Changes in antihypertensive medications in the last 6 weeks * Restless leg syndrome * Average overnight oxygen saturation below 80% * Current use of beta-blockers * History of coronary artery disease * History of Stroke * Atrial fibrillation * Peripheral vascular disease * Suspected cardiac valve abnormality * Ejection fraction \<50% * Type I and type II Diabetes Mellitus (DM) * Asthma or confirmed Chronic Obstructive Pulmonary Disease * Cigarette smoking in the last 6 months * Raynaud's disease * Pregnancy (the normal hormonal changes that occur in pregnancy affect greatly arterial stiffness parameters) If a subject becomes pregnant we will discontinue data collection. * Physically incapable of resting on left lateral decubitus for 40 minutes. * Mastectomy with lymph node removal that might preclude us to monitor blood pressures on both arms