As far as I can guess, it was a badly formatted epub. Is that a thing? Cause I've never had this issue in the 9 months I've owned the reader. I put a new epub in the reader the day before the drain, adjusted the font (something I rarely ever do) and then put the book down. By morning the power was dead. First thing I did when I got it reset was remove said epub. Then I plugged it into the charger. It was fine all day yesterday and this morning I woke up with my reader at 100%. Knock on wood, apparently problem solved.
So are there such thing as badly formatted epubs that can keep the reader thinking all night? I'd come across the concept while trouble shooting my issue. It fits my problem so far cause I never play around with the settings and I never use Wi Fi.