I thought I had been having battery issues with my Kobo when I got it a few weeks ago. I had the same issue of putting the device to sleep at night and waking up in the morning and seeing it's down 20% from what it previously was. Kept happening repeatedly, really weird because the battery hardly ever seemed to move more than a few percentage points after periods of long reading. So instead of recharging it during the day I decided to let the battery drain and charge it from as little as possible.

Around.. 65% of the battery, it seems to have stopped losing charge while in sleep mode, and the battery usage seems to have normalized. It's been a few days and it's only gone down to 50% after periods of long reading. I'm guessing the constant rechargings never let the device calibrate the battery meter, or however this kind of stuff is supposed to work. Still going to let it run out, but it looks like it'll take longer than I thought!