This is simply not true, the 8 hour workday is a complete arbitrary concept. There is a wealth of research to show our creative thinking is best done in short spurts. Time off in between actually helps your subconscious continue to work on the solution, something known as "diffuse mode" when I am attempting to solve a problem. There is a reason why so many of former geniuses, great authors, mathematicians said their best solutions came after a walk through nature.