The human body is amazing; how it can shut down to preserve itself or tap into huge reserves to keep going if that's what it takes.
Years and years ago I cut my finger pretty badly. Not enough for stitches, but it bled really well (and now that I think about it I've cut my finger much worse since then) and as soon as I got it bandaged I just wanted to lay down and sleep for a few hours. Meanwhile you read those news stories about some farmboy who got his arms ripped off in a grain auger, walked back to the house, opened the door with his mouth and then used a pencil in his mouth to push the buttons on the phone (this is before cell phones).
Or yesterday, I had to move a LOT of dirt. More than I was able to. But I did it. And this morning I had to meet someone so I woke up, had plenty of energy, and went and did my meeting. But now, all of a sudden, I just want to go to sleep for about 4 hours. In fact, once I finish here and do my morning contracting work, I think that's exactly what I'll do.