The invention is directed to a positive airway pressure (PAP) system with a head mounted harness assembly with a housing and a head position sensor located within or secured to the housing that detects the position of a patient's head, and communicates this head position information to a controller of the system which may be disposed within the housing having the position sensor or a second housing. The controller varies the output pressure of the pressure source, e.g. a rotary compressor, based, at least in part, on the head position information provided by the head position sensor. In a preferred embodiment, the position sensor is an accelerometer.