I have the Galaxy watch 2 days and i had also the same problem. First time i made reset and everything was fine again. But today happened again. Finally i gound out that the problem was with the goodnight mode. That happened because i was sitting on my desk and Samsung watch decided that i am sleeping! Maybe Samsung wants us to run for 16 hours and sleep 8 hours per day for the watch to work right?
Anyway its a great watch and i hope there will be many updates to correct all bugs and problems.
This has been bugging me for some time too. Seems to me that it only happens when the watch is on my wrist. I bring my hand up to see the time and the face doesn't light up sometimes. When the watch is off my wrist and just laying on a flat surface just lifting it up wakes it up. Checked all the settings but can't see anything amiss.