How the Apple Watch tracks sleep -- and why - CNET

Unlike other wearables such as the Fitbit or Oura, which measure how much time you spend in the various sleep phases and even give calculated sleep quality scores, Apple's sleep tech is more simplified. It just tracks duration of sleep, movement disturbances and heart rate. The content of your sleep isn't analyzed much at all. Instead, Apple's placed a big focus on the time you go to bed and what you do while you wind down.

I think this is a good start for them, as I often look at my sleep data through tools like Autosleep or Sleep Cycle and don't know what to do with it other than go, "looks like I was a little restless at this time for no reason".

My biggest complaint with the new feature is that it doesn't seem to have a way to edit your sleep history. This would be a minor problem, but I've also noticed that it always counts you as getting out of bed at your alarm time, whether you get out of bed or not. I can snooze the alarm a few times and it still shows me getting out of bed at 6AM on the dot. This weekend I slept in all the way til almost 9AM, but the watch says I got up at 6AM. This doesn't feel like a beta bug or anything, but a decision for this release, and I'm not a fan. Submitted feedback saying as much, so fingers crossed this is added.