Random Amounts of Love & Ridiculousness

Your brain is an incredibly energy-intensive organ. It makes up approximately 2% of your total body mass, but at rest it's using 22% of your total energy intake. The only thing that uses the same percentage of energy is all of your skeletal muscle (which is about 40% of your total body mass), with the liver (about 2.6% of your total mass) close behind at 21% (Aragon et al, 2017).

And that's how much energy it's using at rest.

That's the baseline.

So if you're doing lots of intellectual labour, your brain - which already has disproportionately huge energy demand - is going to use even more energy to keep up with the work. That's why you feel so wiped/drained/like you're out of spell slots after high intellectual demand - because your brain is an organ, and you've just done a lot of high-energy work with a high-energy organ.

FANTASIA (1940)