I got my Series 8 in December and everything just magically worked after updating from the Series 3.
Long ago, my wife had set up "sleep" for me so that I'd get an alarm going off automatically every morning instead of needing to go into the clock app on my phone every night to turn the alarm on.
Well... Reading through that article, I saw that they tell a person to set up their "Sleep" (like a focus, or it is a focus, idk lol). I have one of those setup JUST for the alarm but I also have it stopping all notifications that come in during those times. The phone itself will ring 24/7, it has to for my job.
I don't know if a person really has to set one of those up so that the watch knows when to start looking for sleep stuff, but that's why I just posted the link and said that it's chock full of information.
There are some settings in the watch that allow the watch to be used for sleep tracking (From within your watch... Settings, scroll down to Sleep, scroll down to Sleep Tracking, is it on (green)?
The same setting can be found in your phone in the My Watch app. Open the app, scroll down to Sleep and click it. Is "Track Sleep with Apple Watch" on (green)?
Then from within that same screen (in the My Watch app), there are orange letters that say, "Manage Sleep Focus in Settings", click on it.
It opens up the Sleep Focus. Scroll down a little ways until you get to a section titled "Schedule". Do you have a "Next Schedule" showing in a white box?