Not a great one, but perhaps one useful to aspiring mathematicians! My friend, who is now an assistant prof at a major research university, said he could only do ~4 hrs of hard mathematical thinking per day. His colleagues and undergrad advisors all experienced the "four hour threshold." However, he said that Terence Tao was able to stay switched on for most of his waking hours, and that he'd structured his life - with nannies and a house close to UCLA - to take advantage of that. I guess the moral of the story is that normal mathematicians can be productive and quite happy with just a few hours of work per day, but that generation defining mathematicians have to put in longer hours :-)
