Jeff Lliff ted talk on sleep as brain cleaning, and how it's done.
My suggestion is that this brain cleaning carries over to all blood barriers ( brain and spine, parts of the ears, eyes, male testes, joints.
That it correlates to
NREM sleep for brain and spine cleaning
REM sleep for rest of body cleaning of blood barrier areas. (If not how are they cleaned - the same question asked of them as asked of cleaning the brain of waste)
That sleep is not for the brain alone, but includes resetting the digestive system (we can't eat and sleep at the same time), and preparing all body waste out for wake up.