The cognitive ceiling. Research by Ericsson, Mark, and Newport shows that 3-4 hours is the daily maximum for concentrated effort. Beyond that, diminishing returns.
"Diminishing returns" is not the same as zero returns. You'll get more coding done if you work eight hours a day than four hours a day. There's certainly a point where the quality gets so low that the returns are negative (by introducing bugs / technical debt / stuff you have to rewrite the next day), but in my experience 4 hours is not it.
In fact, if the problem is very complicated then it might even take you three hours just to get up to speed with what you were doing the day before.
I mean I'd do it for the good of mankind. Except I wouldn't actually be watching.