I think I understand what you're saying here. One of my friends (currently in his 3rd) told me something similar. He said if I could get in 4-5 of high quality work done (and this is high-quality - no distractions, no reddit, no music, no emails etc.), then he simply calls it a day. According to him - the 6th hour and beyond are just ways to console yourself that you are in the office and doing something. So, once he reaches his pre-defined goals, he simply packs up and goes to the gym or watches a movie.
Of course, he's in his third year and has no 'proof' to show that what he does is the right thing, but it does make some sense if you start thinking that the brain has the capacity to just so much. Beyond a certain point, it simply becomes inefficient to carry on working for 11 hours - especially where mental faculties are involved.