[j-g-faustus]

I have no better knowledge, just parsing the reports: The cleanup is more effective during sleep because the channels that transport waste from the brain expand 60% in volume, and the brain cells themselves shrink by a similar amount to make room.

I suppose that the brain cells can't shrink that much while retaining their full function.

[comrade_ogilvy]

It is not the neurons themselves, but the the glial cells that keep those very large and (potentially) active neurons healthy. Presumably the glial cells must partially shut down in order to physically shrink, thus the neurons must notch down their activity as well.