A split-night sleep diagnostic system is provided for diagnosing and treating sleep apnea in a single night. The diagnostic system includes a blower, a respiratory interface and a conduit connected between the blower and the respiratory interface. Also included is a valve disposed along the conduit. The valve includes an aperture formed in the conduit between the blower and the respiratory interface, and a valve member movable between a first position partially occluding the conduit so that air flows through the aperture and a second position blocking the aperture so that continuous positive airway pressure is provided to a patient. Both diagnosis and titration can be performed using a single system at a single location over the course of a single night.