I woke up one morning to find my watch had about 1% battery left. I thought strage, but put it on to fully charge none the less. I've kept an eye on it since, and found that each day it drops from about 70% to <5% while I'm sleeping. I have already turned off everything I can, and the DND remains in place.
The only change is the Connect mobile app was updated around the same time, I suspect that a bug was introduced that is probably unnecessarly continually polling the watch for data during the night draining the battery.
I would contact my local tech support, but they are still tring to figure out how to set up a chat service.