You are not alone with this problem. Must be some of us that don't fit into their system of tracking sleep. I have the problem with my Watch 6 and also had the problem with my previous Watch 3.
Tracking my sleep just makes me more tired because it claims I am only sleeping 4-5 hours a night which is wrong.
I can get up to use the washroom at say 2AM and my watch claims I don't sleep again until 5AM even though I get right back into bed and back to sleep shortly.
Watch is tight on my wrist also but still is wrong🙄😂
Even the battery dropped 7 percent even though I had the sleep mode and DND on. Kinda getting the buyers remorse on my purchase.
Going to try to reset the watch and see if it helps.
I ha e the same.issues I thought it was something I missed togle on . But no still not tracking my sleep and I have to enter the sleep manually the nest day as my phone asked is I had gone to sleep at certain time