Obstructive sleep apnea (OSA) is a syndrome characterized by repetitive episodes of airway obstruction during sleep, which result in low oxygen level in the blood and bad sleep quality. Both of these effects are implicated in medical, neurological and cognitive disorders in subjects with OSA. The purpose of this study is to examine how OSA affects medical and neurobehavioral outcomes after gastric bypass surgery for weight loss in morbidly obese patients.