I never found the sleep tracking to be useful. I get enough sleep and the watch doesn't tell me anything particularly interesting. I did it a few times and can see my heart rate overnight but it isn't something that changes day to day.
My Apple Watch (see my response above) has lasted well over a day including sleep tracking, on a single charge. It would go to 2 days if I let it. It’ll be able to track your sleep, just turn the always on screen and several notifications off.
I have everything on on my series 6 LTE. After 23 hours on my wrist it usually at 50%. It is new though. But the screen is always on. It seems they have improved battery life on series 6 comparing with my old series 4.
I should have added this is an Apple Watch 6, as well.
It’s definitely reached a point where I’m not worried, so far. Tomorrow I’m going for a run, so that should be interesting.