Everyone's different, of course. Me? I felt sharpest and most productive in my work when I started being able to take a morning off in the middle of the week. I usually work overtime and, after a few days, I start to feel unable to concentrate after the 6th hour of work. When I get something done, I take much more time than I usually would and with stupid little mistakes in it. Just taking a morning off, in the middle of the week, really makes a huge difference for me.
This is just anecdotal evidence but there's something to be said about this. In the end, everyone's different and every job is different. Companies don't have the luxury of knowing all the variables about the way your brain works so they have to find an average. Some people's sweet spots for optimal performance, will fall closer to that average, others wont. If you have the flexibility to find it yourself, great! If you don't, arguing with your boss that you should work less is usually out of question.
Hopefully, companies will become more flexible and work more with its employees brains and less against them.