I use my watch 4 to track sleep, It does an "ok" job but i use "sleep as android" to manually start and finish when i go to bed and when i get up , Because the watch sometimes thinks i'm still asleep/trying to sleep if i don't get out of bed and moving straight away.
If you work shifts at all , It records sleeping in the day as "Naps" and then nags you about it.
Accuracy / Insights wise, i found fitbit better to look at my results , but otherwise the watch does a decent job.
Also just be aware that (and appreciate it doesn't happy to everyone) , you might get a rubbish night of 4-5 hours sleep, but wake up feeling ok, then you look at your sleep log, and immediately feel tired because "i must be tired" .. Easy trap to fall into.