Sleep tracking does not work

Did you set up a sleep schedule? It will only record during that predefined period

Track your sleep on Apple Watch and use Sleep on iPhone – Apple Support

I had been using a FitBit that cost around $100 before getting the Apple Watch 9. The Fitbit recorded sleep whenever it happened including sleep phases, oxygen levels, & body temperature changes, too. The Apple Watch does none of this despite being several times more in cost! I am really regretting my purchase, as none of this was disclosed in the sales materials. Had I known, I would’ve stuck with the Fitbit.

Firstly nobody lists what is not be included in the specs

Secondly the phase analysis of naps is still not convincing science.

Finally look at the unofficial reviews of OS11 on Youtube (they cannot be discussed here)