I had a rather strange issue happen this morning with my new Clara, which is also running KOReader.

I had been reading a book yesterday afternoon in KOReader, with ~95% battery available, and let the reader go into sleep mode with the book still open.

Overnight, the battery had drained completely to zero.

Is that a known issue, or is it more likely something to do with my hardware and not with KOReader?

ETA: I didn't observe this issue prior to installing KOReader, and my settings in the Kobo software were factory default as in Sleep 10 mins, Shutdown never.