Heart Rate Monitors: Optical vs. Chest-Based. Which is better?

Nowadays it seems that almost everybody is wearing an activity tracker or smart watch on their wrist. The majority of wrist based activity trackers will give you information about your step count, your sleep quality and your heart rate. Is an optical sensor accurate? How does it know your heart rate? Is the data that you see on your watch correct? What does resting heart rate mean? We answer your questions.