The modern submarine is not a simple machine. A loss of propulsion, unexpected flooding, or trouble with reactors or weapons can doom a sub crew to a watery grave.
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.
And no one knows that better than military service members and we have the pictures to prove it.