Residency will do that as well. I once remember falling asleep standing in the back of an elevator. But then working 12-12-36 with no days off in-between will do that for you!
When I was in the USN, we came home from a Med cruise, and we had one of our planes that had to have the engine changed.
Everyone else got to leave Norfolk, but a bunch of us had to stay behind and fix the plane. It got craned off, and we had to tow it up a main road to a hangar.
We worked straight through the night to change that engine, and after being up the whole night the evening before that packing on the ship, I think I must have gone more than two straight days with no sleep.
When we were sweeping the hangar in the morning after finishing the engine swap, I stopped, rested my two forearms on top of the vertical push broom, and laid my forehead on top of my arms and closed my eyes.
Just for a second, you know?
I immediately fell fast asleep while standing there, and was abruptly awakened seeing stars and a pain in the center of my brain.
I had fallen asleep, my arms slipped off the broom handle, and my head came down on the end of the broom handle, which went right up into my eye socket. Very nearly put my eye out!