Modern smartwatches come with numerous sensors. Depending on the specific model, they can even measure or estimate blood pressure. Optical sensors for measuring heart rate are now fully standard. In practice, there are hardly any models left without such a function. With many models, it is now possible to measure not only heart rate but also heart rate variability. To measure heart rate variability, the time intervals between individual heartbeats, or, in the case of a smartwatch, more precisely pulse beats, have to be measured. The fact that this has to be a more difficult measurement is shown by a simple self-test: pulse rate can be measured easily with a stopwatch and counting, but heart rate variability is not accessible that way. <\/p>\n