"the great mathematician and physicist Henri Poincaré put in two hours of work in the morning and two in the evening."
"you may be able to work 15 hours a day processing email and doing other less demanding work, but nobody can sustain more than about four hours of intense concentration per day."
"The best data on sustained intellectual activity comes from financially independent authors. While completing a novel famous authors tend to write only for 4 hr during the morning, leaving the rest of the day for rest and recuperation"