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.
I was about 16, helping out with a beach demonstration of operator gadgets at the Navy UDT/SEAL Museum in South Florida, when a SEAL vet gave me and my friends a chance to shoot a full-automatic weapon for the first time: blank rounds from a belt-fed M-60. Each of us stepped up, grunted under the bulk of the machine gun, felt the upward jerk of the muzzle as it burped hot anger toward the surf, and said to ourselves: My God, this rocks.