The human brain is so lazy by default. If I were to compare it to an animal, this visual comes to my mind:
You have to train it, trick it, and bypass its defense mechanisms to take on more work. Otherwise, it will trick you into thinking you are tired, that's all you can, that's above your possibilities, etc.
Humans are highly adaptable to changing conditions. I would say like cockroaches, if that wouldn't feel offensive. 😀
But the default response to potential changing conditions is fightback. Why change? You are so comfortable as you are, you already know all you need to know, no additional effort is needed.
But if the human being faces strong adversity, an extraordinary transformation happens. They have to tap into unimaginable resources, quickly learning new skills and finding the energy to work as much as needed, activating survival instincts, if need be.
It's a bit like the lioness in the image at the beginning. She's resting conserving energy in the tree, but when she or her cubs (if she has any) are hungry, she needs to go hunting, whether she likes it or not, whether she has enough energy for a full hunt or not. Maybe she gets lucky and can steal the kill of a smaller hunter, like a cheetah. Poor cheetah, huh? Doing all the work, spending all that energy and ending up with a few quick bites before the big boys come to the table.
Anyway, I find it remarkable how much a person can change when they are forced by circumstances, and the brain can't play tricks on them any longer.
To some degree, it seems these mind games are only possible when someone hasn't reached a point where hard decisions and actions need to be taken or else...
In a way, that's probably a good indicator. If your mind still persuades you that you are too tired, it's too difficult, you can't do it, you probably haven't been with your back against the wall, and like the lioness, in the situation to hunt or die. Civilization spared many of us from a day to day existence, where if you don't hunt, you die. But that also comes with a numbness of the mind, which tries very hard to elude the prospect of more work.