Can I rely on my Apple Watch Sleep Data?

https://www.healthline.com/health/how-much-deep-sleep-do-you-need
The above is a decent guideline for you to print out and to keep on your bedstand or nearby for reference. Note that there will always be plenty of variance from night to night, with some stages seeming to get short shrift, especially deep sleep.

As for the app, I can't comment knowledgeably because I wasn't a designer or a tester and don't know how accurately the watch can assess each stage. I use a Samsung Galaxy watch that does the same thing, and I take it as a good indicator of the quality of the previous night's sleep. When I have a crappy night, I shrug and hope to do better next time. Often, that's exactly what happens....unless I'm stewing over something that requires me to get it done.

I use a Cpap machine and have an Apple watch as well. Neither is perfect. The machine records use time so it it will take into consideration time awake whereas the watch appears to know when you are awake or asleep as well as your levels of sleep. I read that REM sleep ideally is 25% however that becomes less as you age. the watch also notes breathing disturbances. Wishing you good health and sweet dreams