I've been thinking about buying a cheap Apple watch just as a sleep tracker.
Sleep tracking was important for me too, but I've solved that by having whoop (it was a gift, though). It's tracking that stuff nicely, and I only have to charge it once in 1-2 weeks.
You're probably better off with a cheap Garmin for that. The battery will last a month instead of a day. (Or multiple months if you only wear it at night and turn it off every morning)
Apple watch starts at $250, Fitbit does sleep tracking too and starts at $70.
Do you already sleep with a wrist watch on? I got one for the same and realized I can't stand sleeping with something on my wrist. Also realized if I want to use it for anything else throughout the day, that's the best time for me to charge it.