definately take a break, relax, watch tv, eat or whatever if you're brain isn't working - i mean, it's not going to go in then, what's the point? and if you feel like ur wasting time not doing ur work, then do something productive that doesn't require brainwork - like for example typing up notes on the computer than u handwrote or something, or like when i write my syllabus notes, most of it doesn't go in my head anyway, i just write and afterwards i read it - so u cud do something like that. the main point is to take breaks during work, like every 30 minutes take a 10 minute break, so ur head isnt overloaded.