Benjamin Franklin nailed it when he said, "Fatigue is the best pillow." True story, Benny. There's nothing like pushing your body so far past exhaustion that you'd willingly, even longingly, take a nap on a concrete slab.
When Gen. Dwight D. Eisenhower issued a letter to his troops just before the D-Day landings in Normandy in 1944, he told them, “I have full confidence in your courage, devotion to duty and skill in battle.” He was writing to American troops who had to meet these minimum physical requirements: seven pull-ups, 31 jump squats, 27 push-ups, 52 sit-ups, and a 300-yard sprint in 52.5 seconds. Soldiers — all of them — had to be fit in 1944.