I first read about the ~4 hour limit from D.E. Littlewood's book, Littlewood's Miscellany. He recommends "four hours a day or at most five, with breaks about every hour (for walks perhaps)."

Since then I've read of luminaries mentioning this in passing, but I haven't really tried to employ it. (I'm also pretty sure I've seen pg say this is empirically false with YC founders, and perhaps he and axiom are correct)

I'd like to know if there's evidence of this for learning, however, even if it is mathematics.