Your skepticism is justified. Oura publish their data, and to quote them:
>Oura ring was 96% accurate in detecting sleep compared to polysomnography, 48% accurate in detecting wakefulness, 65% agreement in detecting “light sleep,” 51% agreement in detecting “deep sleep” and 61% agreement in detecting REM
So it's basically wrong with sleep categorization about half the time.
From the study: "Most devices (Fatigue Science Readiband, Fitbit Alta HR, EarlySense Live, ResMed S+, SleepScore Max) performed as well as or better than actigraphy on sleep/wake performance measures".
The Garmin watches have been replaced by newer versions with upgraded sensors. Garmin's tracking isn't perfect, but each version has made small improvements.
Many have already pointed that there have been studies to measure the accuracy of sleep analysis by wearables. They aren't that great. If you have a concern, you should definitely seek out a polysomnograph, and wearables should not be a replacement for that. However, you can't get a polysomnograph every night, and wearables are probably the best option for regular analysis. Just because they're not that great doesn't mean they're useless, and doesn't mean they won't improve.
Most devices (Fatigue Science Readiband, Fitbit Alta HR, EarlySense Live, ResMed S+, SleepScore Max) performed as well as or better than actigraphy on sleep/wake performance measures, while the Garmin devices performed worse.
Can you cite a medical study that evaluates the accuracy of your smart watch’s assessment of your quality of sleep?
Preferably one not sponsored by the manufacturer?