[amelius]

But why didn't our brain evolve to take the debris out while it s working?

[pstuart]

Unqualified anonymous internet answer: it's probably too busy.

I believe the brain actually shrinks slightly and channels expand to aid in the flushing.

Anectdata: note how when you're really tired you can't think clearly? Also, how (apparently) if one goes without sleep for too long then the brain suffers irreparable damage.

[Khanthulhu]

But what if we evolved sleep before we evolved brains? Might it then have a different benefit and the brain ones came later?