A system and method of tracking activity includes a motion sensor, a light source and a light detector. The light detector is configured to capture an amount of the light that is reflected back to the light detector, at least a first portion of the light reflected back to the light detector is reflected from a blood vessel disposed under a skin adjacent to the housing. A processor is in communication with the motion sensor and the light detector and can process the reflected light to identify heart beats and produce an indication of a heart rate. The indication of the heart rate can be displayed on the display screen as an option, in addition to the metrics that quantify the motion data.