Is it on your first page after waking it or later in your reading?
If it's the 1st page, there is currently a bug/feature related to the sleep cover that means if you let it auto-sleep (even if you don't have a sleep cover), when you wake it it will auto-sleep after a short period of time (before you get to the end of the page) unless you change the page or bring up the bottom menu before it times out it goes back to sleep. Workaround is to either sleep it with the switch or touch screen before it times out (or read really fast on your first page
). This is discussed/debated in other threads and is meant to stop battery drain if sleep cover accidentally opens.