An optical sensor for a monitoring device for monitoring the vital signs of a user is disclosed herein. The optical sensor preferably includes a photodetector and a two light emitting diodes (“LEDs”) that emit green light. The optical sensor includes a light barrier that includes multiple walls that block any non-reflected light from the LEDs from transmission to the photodetector so that the photodetector only receives light reflected from the artery of the user resulting in a more accurate detection of the heart rate of the user. A sensor signal from the optical sensor is processed with an accelerometer output signal from the accelerometer to generate a real-time vital sign for a user.