A sleep study that includes both a diagnostic test that lasts 2 to 4 hours and a therapeutic test that lasts the same night. Every adult is eligible for this study, allowing for the immediate treatment of patients with severe OSAS. The study is conducted as an NPSG if the patient is not “split” to CPAP. If the diagnostic test reveals signs of sleep apnea and treatment is recommended, the patient may return for a full night of CPAP therapy.