I traveled a few hundred miles today and was perfectly alert while I was doing it, then came home and within 15 minutes was absolutely exhausted - NOT because home is exhausting but because my brain knew I was at a place where being exhausted was okay.
@scalzi I once saw a car hit a motorcycle, sending the rider hurtling through the air. He landed in the middle of the intersection, immediately leapt onto his feet, ran to safety on the sidewalk, then hurled himself down on the ground, all on autopilot and all in the course of a few seconds.