Smartwatches tend to rely on a technology called photoplethysmography or PPG. It is a special system where a green light is projected on the skin hundreds of times per second with the reflection of the light being captured. With more light being reflected between heartbeats, this acts as an indication for heart rate and thus, it is measured. ‘Optical heart rate monitor’ is the common hardware that is used for this system and it always helps you gain access to results. While PPG is known to be a good indication of heart rate, you also need to know that it is not 100% accurate.