Why has my Apple Watch stopped recording sleep data?
Watch was recording and updating sleep data and then started to be intermittent then stopped completely
[Re-Titled by Moderator]
Question marked as
Did you try to force restart your watch and the paired iPhone?
To force restart the watch, hold down the Digital Crown and the Side button at the same time for at least 10sec, until the Apple Logo comes back again on the screen.
Force restart iPhone - Apple Support
If still not working, unpairing the watch and connecting it again would be the next step:
Unpair and erase your Apple Watch - Apple Support