My anecdotal experience has been that short naps between reading works brilliantly in terms of retaining the concepts. I always used to take a nap after reading for about 40min - 1hr in my college days.
I have had the same experiences. After a given amount of hard studying (anywhere from an hour to a few hours), I find that I cannot maintain focus anymore. A short nap always fixes the problem, and when I come back to it things usually stick much better.
