John D. Cook, writing about a potential limit on human concentration of about four hours:
Apparently this is a common pattern. Cal Newport mentions this in his interview with Todd Henry.
Now we also know that if you study absolute world class, best virtuoso violin players, none of them put in more than about four or so hours of practice in a day, because that’s the cognitive limit. And this limit actually shows up in a lot of different fields where people do intense training, that you really can’t do about more than four or so hours of this type of really mental strain. And they often break this into two sessions, of two hours and then two hours. So there’s huge limits here. I think if you’re able to do three, maybe four hours of this sort of deep work in a typical day, you’re hitting basically the mental speed limit, the amount of concentration your brain is actually able to give.
This doesn’t mean a person only works for these few hours. No doubt, I can do “stuff” for eight hours or more in a day. Throughout my educational career, I have been able to maximize my efficiency by optimizing the tools I use. However, I find the time in which I can get lost in complex problems is on the order of four hours. Beyond that, my mind feels taxed and I lose interest. Unlike many mentioned by Cook, I prefer those hours in the evening or late at night.