These smart watches have many functionalities of your phone, but they have added sensors to track your various activities, they can continually monitor your heart rate, and some of them are even able to calculate VO2 max, which is your aerobic capability. Thus by monitoring your heart and respiration, they can even make some major predictions about your health and well-being. Many of these sleek and smart gadgets can identify when you are sleeping, walking, jogging, running, swimming or climbing stairs. Thus accurately calculating the energy expenditure during the day. Some of them are even able to count the number of reps done in a particular exercise! What more could you ask for? These gadgets are quite reliable as they are being offered by well-known companies like Apple or Garmin.
If you want even more, then wait, new watches that can continually monitor your blood glucose level are already being launched(4).