Sorry but anyone who has worked in a professional high-tech environment should be aware of that an 8 hour days is not productive. You NEED to spend you entire day on this, you mind always needs to be on it to work on that level.
Einstein would certainly not have solved relativity by saying "Uh I guess from 9 to 5 I will work on this with a one hour break in between and then I am heading off to the fun stuff called leisure time".
No... Only if you think about these problems day in and day out you can work at in the top percentiles and this is the kind of people that get hired there.