I have been experiencing this same issue for many months with my watch 6. I have tried everything, so I decided foolishly to purchase the watch 7 thinking it may have been resolved with this model, but sadly no!
for the first few days it recorded nothing for the "sleep" then it might record for a couple of hours in the middle of my sleep, sometimes with several gaps and now it has stopped recording again.
I emailed Samsung Service with my issue and to date I have not received a reply.
My expectations have dropped for Samsung products.
