Why is that so important？ Because fatigue accumulate with astonishing rapidity. The United States Army has discovered by repeated tests that even young men—men toughened by years of Army training—can march better， and hold up longer， if they throw down their packs and rest ten minutes out of every hour. So the Army forces them to do just that.
Your heart is just as smart as the US Army. Your heart pumps enough blood through your body every day to fill a railway tank car. It exerts enough energy every twenty-four hours to shovel twenty tons of coal on to a platform three feet high. It does this incredible amount of work for fifty， seventy， or maybe ninety years. How can it stand it？ Dr. Walter B. Cannon， of the Harvard Medical School， explains it. He says， “Most people have the idea that the heart is working all the time. As a matter of fact， there is a definite rest period after each contraction. When beating at a moderate rate of seventy pulses per minute， the heart is actually working only nine hours out of the twenty-four. In the aggregate its rest periods total a full fifteen hours per day.”
A physical worker can do more work if he takes more time out for rest. Frederick Taylor demonstrated that while working as a scientific management engineer with the Bethlehem Steel Company. He observed that labouring men were loading approximately 12.5 tons of pig-iron per man each day on freight cars and that they were exhausted at noon. He made a scientific study of all the fatigue factors involved， and declared that these men should be loading not 12.5 tons of pig-iron per day， but forty-seven tons per day！ He figured that they ought to do almost four times as much as they were doing， and not be exhausted. But prove it！
Taylor selected a Mr. Schmidt who was required to work by the stop-watch. Schmidt was told by the man who stood over him with a watch： “Now pick up a ‘pig’and walk... now sit down and rest... now walk... now rest.”