If that's true, that might also be part of why dreams exist. If the glymphatic cleanout during sleep is a rhythmic constriction wave across the brain, then whatever the brain is doing at the time the wave crosses it becomes a sustained thought as a side effect.

Maybe that's why sleep, in the sense of becoming immobile, is a thing. The brain disconnects a lot of the motor control so that the hallucination caused by sustained randomness from cleanout waves don't make us flail all over the place and hurt ourselves.