geniuses: the brightest minds spend their time either pursuing an activity with ferocious intensity, or engaging in complete restoration and recovery. This approach, Csikszentmihalyi discovered, not only prevents creative burnout and cognitive fatigue, but it also fosters breakthrough ideas and discoveries

Then, from about nine to one, unless he was giving a lecture, he worked at his own mathematics. Four hours creative work a day is about the limit for a mathematician, he used to say.

diminishing returns after three or four hours.