If your day is only 50% productive, then clearly working only 4 hours a day would make you 100% productive! Only it doesn't work that way. Unless you have laser

If your day is only 50% productive, then clearly working only 4 hours a day would make you 100% productive! Only it doesn't work that way. Unless you have laser focus, most people can't pick and choose the times that they are productive. Rather, they show up for 8 hours and warm up getting into things, then are productive for a while, get interrupted, think about that whole lunch thing, eventually get back into a productive mood, then gradually wind down and think about leaving work. Net result might be 4 hours of productive work, but it's interspersed with unproductivity.