The brain is a very delegate things in our body which deals with a lot of things at once depending what you delegate it to do.
I agree with you on the explanation you gave concerning the brain which is exactly they way it works because when over laboured it takes time for a good reason to be performed again.

When you mentioned WILL POWER which you said it serves as battery cell to the brain, it now makes me to understand the way I feel sometimes when I want to perform a task. I got to understand that the more works is been performed by the brain the more the body gets tired too because energy is been used which shows that the more at some point the brain needs rest.

I think sometimes we have to protect the Brain 🧠 from too much work load because getting it over laboured is not advisable though I know that is not our fault to work with pace with the brain and let it function well. Brain fatigue is basically not good because it ways the more it weakens the more disturbed we get health wise. As you said is not physical but internal fatigue so lets do our best to less brain stress.

We should all learn to space what we do and find a way to reduce stress that weighs down the will power because when the will is down we do not have strength physical to do the work and that's what makes us become lazy sometimes.
I guess we have to lean to do more in the morning and learn to rest in the evening to ease the brain