One thing I've learned during exam weeks is that I generally can't think intensively for more than two hours at a time. The efficiency of my studying goes down if I don't take decent breaks between two-hour blocks, and sometimes I can't even think enough to play video games after especially mental sessions.
It could be my imagination, but this effect is unfortunately noticeable during the two-and-a-half-hour exams that come at the end of each semester. I charge in at the beginning of a test with all the knowledge I've gained and hope that I get through all the questions before fatigue sets in. It's worked so far.