The thing I say about all of them is...what's the value in you having better tracking?
Let's say it's off by 4% and that you got 4% less deep sleep last night than your tracker says? What are you going to do about it? You can't change the data from the past anyway. They're all "good enough" I'd say.
The value of accurate tracking is comparison. As someone who suffers from chronic sleep issues, being able to accurately compare the effect of interventions on my sleep pattern, as well as account for seasonality etc is invaluable. Accurate tracking also helps forestall habituation - it's like keeping a paper diary, obviously you can't change the past (no one ever suggested you could) but you can make yourself aware of large changes that happened too gradually to be obvious.
Yeah I'm not sure what I'd use this for. I tend to know if I've had a bad night's sleep, and the interventions I'd try are all things I should do anyway - don't drink, less caffeine, lower stress.
The Oura ring does a lot more analytics. There's a "sleep score", a "readiness score", various algorithms to suggest an "optimal sleep time", etc.
The Apple Watch just records stuff and throws it in the Health app. That's it. They added a "vitals" app that mostly alerts you when the numbers it records are drastically different than the norm, but it's not really analyzing or suggesting anything.
I get the feeling that Apple thought there would be a whole ecosystem of apps that would analyze your sleep data, so they didn't need to build any of it into the watch or the health app itself.
