Apple built in sleep tracking to watchOS 7, but they took more of a hands off approach. I used it for a few months, but gave up cause it was just lacking too much info and added more annoyances then benefits.
I highly recommend just using Apples new shortcuts feature that allows you to change you watch face at certain times of the day or based on locations. I use that with also setting theater mode that turns off the screen and allows the user to set the brightness based on the scrolling up the dial. I then use Sleep++ to track my actual sleep it give me a more detail look at how well I actually slept.
My other video on using shortcuts to change the watch face automatically on the watch: https://www.youtube.com/watch?v=m_nXha4PObA