Studies have shown a link between mental and physical exhaustion that means being mentally burned out causes you to physically burn out more quickly. Despite that your heart and muscles could continue, your brain convinces them to give up when it’s already mentally ready for a break. Your perception of how much effort is involved is more important than the straightforward reality of how much you could actually physically accomplish.
All of this shines an uncomfortable light on the squishy mutability of the divide between physical reality and mental perception. By that I mean: What divide? Where? Each of us is stuck here looking out from between our own ears, and our reality is made up of what we believe we perceive. If we believe we’re tired, we can’t continue. If we believe we’re in love, we’ll do ridiculous things and be happy about it. If we hold our nose, bad milk tastes like nothing at all. If our reality is our everything, and our reality is an unreliable, subjective thing, reality is equally unreliable except as a philosophical idea.
We have a particular literary term for when a fictional character demonstrates the same thing. As we are all heroes in our own lives, so we are all unreliable narrators, too.