Aurora: Looking under applications I saw that "mobile network" was "on." But I don't think this could have been the culprit because I had never turned the mobile network off before, and never had the overnight battery drain take place. This makes me curious though--will having that setting set to "on" make the battery drain more quickly, even if one is not using the mobile network?
Bookworm Girl: that's interesting what you say about the file. The night before switching it off I was reading a epub document I had converted from pdf. I noticed for some reason the Sony was having difficulty with this document. When I would highlight passages it was moving very slowly, and sometimes would end up flipping to the previous or next page instead of highlighting. Finally it just refused to highlight at all. I wonder if there could be something about the formatting of the document that the Sony has some trouble with, and which was making the processor work extra hard. Would still be surprised if that caused the battery drain though, because I only spent about an hour reading that document--and the other mysterious thing is the battery icon did not show any drain--all four bars were there before I turned it off.
One possibility that just came into my mind: if the eReader is in sleep mode, but something is making contact with the touch screen, could this cause battery drain? Would the processor keep firing if something was making contact with the screen, even if the reader was sleeping?