Bismar, I think sleep mode is the status the Kobo goes into after nothing is pressed for 10 seconds. So if you take 30 seconds to read the text on the scree 20 seconds of that could be in "sleep mode".
Don't quote me on the time spans, it might be 10 min. I'm just pointing out the difference between sleep and off.
1500 pages in one sitting, must have been a good book, or books.