First check if you have set "Battery saver" in Sleep mode and if you do disable it.
Second, try to set up sleeping window from 01:00 till 09:00 for every day.
If this 2 options do not help, then it is time to first make a full backup of the watch ( settings/sistem...) and then "delete all data and factory reset" (settings/sistem....). If you have some applications and watch faces instaled you will have to reinstal it...
"DELETE ALL DATA AND FACTORY RESET" resolved almost all my problems (sleep and nap detection, HR accuracy, altitude accuracy.... it is preaty good now)
