Lucent says:
[0011] The sleep detector may comprise an electronic camera for forming images of the viewer, and pattern recognition means connected to the electronic camera to monitor the physical condition of the viewer. For example, the pattern recognition means determine whether the viewer’s eyes are open or shut. The apparatus may further include logic means connected to receive output from the pattern recognition means to distinguish normal blinking from the onset of sleep.[0012] Alternatively, the sleep detector may comprise a device wearable by the viewer for monitoring the physical condition of the viewer. For example, the wearable device may include one or more of an accelerometer, a heat flux sensor, a galvanic skin response sensor, a skin temperature sensor and a near-body ambient temperature sensor.
[0013] The sleep detector may be connected to the digital video recorder via direct electrical connection or via a remote-control type interface.