I have found it helpful to pace myself when I have a big study assignment. If I had to do a lot of reading, I'd read, underline, take some notes in the margins, make notes on paper, for about an hour or so, maybe only 15 minutes at a time, then get up and take a short break, have a snack or do some exercise. Then go back to the studies. If it was particularly difficult subject matter, I found I had to see it (read it), say it out loud (read it out loud maybe several times), and write it down verbatim. It is tedious to have to do that, but it was the only way I could retain difficult material.