[draven]

Not OP but I have a Fenix 7x. What happens very infrequently is that I would wake up during the night, and then be absolutely still (I'm trying to go back to sleep) except for checking my watch every 15-20mn. This does not get detected as "awake" the following morning.

I don't really care because I got this watch so I can upload a track and follow it while recording various metrics during 5+ days, and for this it works perfectly.

[coredog64]

There are two data points you need to accurately track sleep: Motion and respiration rate. With that information you can identify light sleep vs. deep sleep, and sometimes you can differentiate between light sleep and being still.

Source: Worked on the ResMed S+ and got a theory dump from the PhDs running the project.

[mandevil]

Sleep versus awake from a sensor on the wrist is always going to be subject to wide error bars.

Source: worked on an experiment trying to make people fall asleep while driving (in a simulated environment so it was safe- I built the simulated environment). It took a lot of EEG wires and gel in subject's hair etc. to accurately figure that out.

[draven]

I would think that "pressing the button to turn on the backlight so I can check the time" would be a good indication that I'm awake :)

[MrMetlHed]

Same experience for me. If I wake up but lie still it's not nearly as accurate. What I do find useful is the Body Battery score, that generally maps well to how I feel the next day. And the Stress Score is an eerily good indicator of when I'm getting sick and when the illness is stopping.