 Participants in this research study will undergo two overnight sleep studies PSGs, pulmonary function test, and CT scan of the upper airway and chest. The first sleep study will evaluate the sleep breathing disorder and the second sleep study will measure the upper airway collapsibility, called critical closing pressure Pcrit. Pcrit is measured by a modified continuous positive airway pressure CPAP machine which can provide a wide range of pressures between 20 and -20 cmH2O in order to modify upper airway pressure. 