A wearable electronic device, comprising a heart rate sensor to determine a heart rate of the user, a motion sensor to detect motion of a user, the motion sensor operated in an initial calibration phase during which the heart rate sensor is detecting the heart rate of the user, and the motion sensor is detecting the motion of the user, the calibration phase being used to compute a correspondence between motion of the user and the heart rate of the user, the motion sensor being operated in a low power phase during which the heart rate sensor is powered down, and a processor configured to run a heart rate estimation module configured to calculate an estimated heart rate of the user based upon the motion of the user and the correspondence between the heart rate of the user and the motion of the user determined in the calibration phase.
