I own a Forerunner 935, Vivosport and a Vivosmart 5. I seems that the newer the model, the higher the stress readings. I do not have a physically demanding job but if i wear the Vivosmart 5 all day (on a typical deskbound office day), my stress levels read up to 80. While on the days that I wear the Forerunner, the stress levels vary between 15-25.
Although the stress levels are probably nothing more than indicative, I'm not so sure what number to rely on anymore.
I wonder if the the HRV measurements are getting more accurate whenever there is a new model?