Originally Posted by WarGhSt
Had this happen a couple of times. Turned out the case I bought was touching the screen when I had the device in standby, (showing the cover of last read book) triggering the IR sensors I guess? Unfortunately I like the case so I just decided to start shutting the PRS-T1 off every night.
Try adjusting the settings so that you have a blank screen in standby and see what happens. Turning the device on every day is going to lead to more recharging of the battery and therefore reduced battery life.