A posture detection system includes an implantable device and a patient-external respiratory therapy device coupled via a communications channel. At least one of the implantable device and the patient-external respiratory therapy devices includes a posture detector. Posture information is transferred between the implantable device and the patient-external respiratory therapy device. The posture information may be used in connection with sleep detection or to modify therapy delivered by the implantable cardiac device and/or the patient-external respiratory therapy device.