An in-ear optical sensor device sits deep within the ear canal of a subject such that is in the cartilaginous region and/or the bony region of the ear canal where effects from temporomandibular joint activity or other movement, and external light, are limited. The device has a lateral housing and a medial articulating head joined at an articulation joint so that the medial articulating head may be angled relative to the lateral housing to help the device fit within tortuous ear canals. The device also includes a seal configured to conform to the shape of the ear canal to physically support the device to further limit movement and block environmental light. An audio receiver is also included in the device.