I just use the native sleep tracker in the watch. I find it is pretty accurate for me after using it for months now. Ever since Apple added the detailed sleep tracker to the watch os update I haven’t found the need to play around with tracking apps.

I set my sleep schedule to start at 11:30 and end at 9 am with NO alarms set (I’ll set my own on my watch, they still work regardless of the mode) and it tracks it every night without fail. Even if I got to sleep a little early sometimes it realizes I’m passed out and will record that data too. I’ve even had it record a quick nap at 9-10pm then I got up for an hour, then off to bed at 11 again lol.