I spend hours either poring over books or on the computer, and found I was starting to see everything blurry once I looked up. I had things checked out, and it seems that as we get older, our eyes don't change focus as quickly, so the longer we spend at one focus, the more likely it will take some time for the opposite focus to work well.
I was told to do similar to Synthetic and Clear_Prop, particularly looking into the distance, at every opportunity. So I started studying next to a window, and as I sat up, I looked out into the distance, then back to the books or computer. It has more/less stopped the problem.