Some embodiments provide a wearable fitness monitoring device including a motion sensor and a photoplethysmographic (PPG) sensor. The PPG sensor includes (i) a periodic light source, (ii) a photo detector, and (iii) circuitry determining a user's heart rate from an output of the photo detector. Some embodiments provide methods for operating a heart rate monitor of a wearable fitness monitoring device to measure one or more characteristics of a heartbeat waveform. Some embodiments provide methods for operating the wearable fitness monitoring device in a low power state when the device determines that the device is not worn by a user. Some embodiments provide methods for operating the wearable fitness monitoring device in a normal power state when the device determines that the device is worn by a user.