Wearable electronic devices, systems, and methods for monitoring sleep are described. In one method, motion sensor values of the current motion of the electronic device are acquired and the change in motion intensity of an electronic device is calculated by comparing motion sensor values over two or more time periods. If the change in motion intensity fits a predetermined pattern, it is determined whether the electronic device is currently being worn. If it is determined that the electronic device is currently being worn, sleep monitoring is performed.