I'll bet you thought that if you were mentally exhausted it was because of all the energy your brain just expended. Turns out it's probably not so; rather, what you believe makes it so. From Scientific American:
Unlike physical exercise, mental workouts probably do not demand significantly more energy than usual. Believing we have drained our brains, however, may be enough to induce weariness.
Let me place this in context. Atheistic materialism is committed to the theory that everything about our minds is explained by the physical activity in our brains. What we think has absolutely no effect on our physical selves; it's always the other way around.
It's an absurd theory on many levels, and this article reveals one more reason to reject it. It notes that difficult mental workouts are associated with stress in other bodily systems, which might also explain the weariness, and yet not all of it. Attitude matters:
Equally important to the duration of mental exertion is one's attitude toward it. Watching a thrilling biopic with a complex narrative excites many different brain regions for a good two hours, yet people typically do not shamble out of the theater complaining of mental fatigue. Some people regularly curl up with densely written novels that others might throw across the room in frustration….
Such fatigue seems much more likely to follow sustained mental effort that we do not seek for pleasure—such as the obligatory SAT—especially when we expect that the ordeal will drain our brains. If we think an exam or puzzle will be difficult, it often will be.
If materialists are right, and everything about the our mental states is explained by physical causes, then how is it that in this case our physical state is partly explained by our mental states? Why does belief affect our bodies?
I expect some will answer that the attitudes that cause mental weariness are themselves entirely the result of electrochemical activity in our brains. That doesn't resolve the issue. Either attitudes (beliefs, ideas, etc.) have effects or else they do not; and if they are entirely the product, the “epiphenomena,” of electrochemical activity, then it's only the electrochemical activity, not the attitudes or beliefs, that has any effect.
It's further evidence of materialism's multi-level absurdity.