Heart rate flatlining during sleep - causing wrong sleep time and sleep score

In May 2024 I bought a FR 255 Music, everything was working well except my sleep tracking. I noticed that the watch often registered sleep 3-5 hours late. As I really put Attentation to my sleep and revovery, this really bothered me. A week ago I received a new FR 255 Music, as the Support looked into my data and screenshots and said, that my HR sensor is faulty. You could see that it Was flatlining during the night and sometimes it fluctuated to much. 5 days went by with the new watch, first 4 nights were measured exceptionally well. This night the same problem accured again as shown in the picture and it is really frustrating. I tried everything: tighten and loosen the watch, wearing it higher and lower, problem seems to persist. I also dont think that I sleep on my arm or something. This problem with faulty HR readings only happens during the night, during the day HR seems fine. Does anyone have a solution? How high up / tight do you guys who regularly have correct sleep readings wear the watch? Chances must be near zero that both watches have the same problem.

Picture: My watch detected sleep at 4 pm but I corrected the bed time so that you can see the HR Problem.

  • Look for discussions about the issue of gaps in HRV measurements at night. People note that they only occur in people with fairly low resting heart rates, around 35 bpm.

  • I already did before posting this. Seems like garmin identified HRV Gaps as "restricting blood flow because of laying on the wrist", yet people dont believe it. I myself make sure not to sleep on my arm or wrist.

    My RHR during sleep is 47-53, so this shouldn't be the problem.

    From now on I Follow the Suggestion of a garmin Youtube Video on how to wear the watch. I will wear it just above the wrist bone and fairly loose, lets see if this will help. The most plausible reason my heart rate looks so weird and "stuck" is that I might be laying on it. I've also never seen this Problem from anyone else.