An in-ear hearing device includes a light source configured to emit light, a photodetector configured to detect the emitted light after the emitted light passes through tissue of a subject, a spout; an audio receiver configured to deliver a sound to the subject through the spout, and a dome configured to conform to a shape of a subject's ear canal when the hearing device is in the ear canal. An output of the light source and an input of the photodetector are separated by the dome, and the dome absorbs and/or reflects at least part of the emitted light. The photodetector may be a forward biased photodiode. The sensor device can be realized with power levels, circuitry components, and in package sizes, of hearing devices.