Studying is actually hard when the student is unable to concentrate on what he or she is doing.
Reading comprehension is likely to be difficult when you have a poor concentration. And sometimes, even if we managed to concentrate during the first hour of our studies, it seems that our concentration diminishes over time.
And fading concentration is very possible if you have been sitting for several hours. Our steady sitting position affects our blood flow and poor blood flow badly affects our concentration. So the best thing to do is to revive your diminishing concentration is to stand-up and shake your legs! Shaking your legs will get the blood flow back to normal.
So the next time you feel that you can not concentrate anymore to what you are reading, give your leg a shake!