A:Answer I also had similar problems it measures only 3 to 4 hours after 1 am. Disabling Breath monitoring (beta) made a difference and it started measuring full sleep. Yesterday I enabled breath monitoring again it starts measuring 3 to 4 hours only.
I have disabled again and I will check today. I purchased the watch 10 days before. First 2 days with default settings it was working fine, then I enabled some more settings like breath monitoring etc, which started this problem. Customer care could identify, they simply wanted my watch for service. However after resetting and leaving it in default mode it started working. You may try that.