Since 1947, the U.S. military has relied on a system called “up or out” in managing its personnel. While the severity with which it has been applied has swung, pendulum-like, as the nation has repeatedly built up and drawn down the military, the basic idea has remained the same: Get promoted to the next rank within a certain timeframe or be separated from the service.
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.