" The fact is that your productivity begins to decline after eight or nine hours of work. For this reason, working long hours into the night, although it is sometimes necessary, means that you are usually producing less and less in more and more time. The more tired you become, the worse the quality of your work will be and the more mistakes you will make. At a certain point, you can reach “the wall” and simply be unable to continue, like a battery that is run down. "

― Brian Tracy , Eat That Frog!: 21 Great Ways to Stop Procrastinating and Get More Done in Less Time