[logfromblammo]

Have you ever dug two or more graves for one dead person, then filled in the ones that didn't actually get used for the interment?

Probably not. You wouldn't want to waste time on back-breaking labor that has no productive purpose, would you?

But the white-collar desk job does that every time it says "work 8 hours every day" while the worker consensus is that the 7th and 8th hour is mostly useless, and subsequent hours in the same day are often counterproductive.

If you spend 8 hours digging a grave by hand with a shovel, your muscles get tired. But if you do it long enough, the muscles get stronger, and you can dig graves for longer before getting fatigued. If you spend 8 hours a day thinking about difficult problems, your brain gets tired. If you do it long enough, you get smarter, but also burn yourself out, go slightly insane, and lose the ability do the job at all any more.

They're different kinds of cells. They have different physiological processes for maintenance and repair. Muscles don't sleep and flush with CSF; brains don't bulk up and add additional nuclei.