Wearable heart rate monitor

A wearable fitness monitoring device comprising:

• a motion sensor configured to provide output corresponding to motion by a user wearing the fitness monitoring device;
• a photoplethysmographic sensor comprising (i) a periodic light source, (ii) a photo detector positioned to receive periodic light emitted by the periodic light source after interacting with a user's skin, and (iii) circuitry determining a user's heart rate from an output of the photo detector;
• and
• control logic configured to:
• (a) operate the heart rate monitor in a first mode while also operating in a second mode configured to detect near proximity of the wearable fitness monitoring device to a user's skin, wherein the first mode is configured to determine one or more characteristics of a user's heartbeat waveform when the wearable fitness monitoring device is in near proximity to the user;
• (b) from information collected in the second mode, determine that the heart rate monitor is not proximate to the user's skin;
• and
• (c) in response to determining that the heart rate monitor is not proximate to the user's skin, end operating the heart rate monitor in the first mode.