[rauschs]

Es liegt wohl nicht an der Uhr - gefunden:

Most watches, arm bands, and fitness heart rate trackers are designed for heart rate only. This means that they do not accurately measure R-R intervals. R-R intervals are the exact time measurement in milliseconds between each heartbeat that are needed for truly accurate Heart Rate Variability (HRV) calculations.

Due to the limitations of those hardware devices, we generally do not recommend them for reliable HRV readings. These devices are fine for just getting your basic heart rate though!

...the best option is to invest in a chest strap.