Unfortunately Samsung doesn't get the sleep tracking right. It registers far too little deep sleep and there's better performance around. I really dont know why Samsung hasn't nailed this and I'm disappointed with the sleep tracking. It was farbetter on my active2 watch.I bought on pre-order, and I really hope Samsung sorts it out through updates.
I was also disappointed at it, as it was one of the reasons I decided to get the watch 4. Unfortunately other main reasons are also not being matched, I haven't been able to install Samsung Pay (neither on my phone or watch).