I’m convinced Rapid Eye Movement is a type of mechanical pumping action that directly affects the brain. Either the eyeballs themselves, the thrashing Optic Nerve, or both.Perhaps creating small amplitude waves that help flush brain waste out into the lymph system.
Recall the eye sockets directly impinge on the brain pand, including some nearby very critical glands. And the Optic Nerves move back and forth quite a bit back there.
REM happens for a reason, and in animals, too. Occam’s Razor says the simplist answer is often correct. Itty bitty mechanical brain bilge pump. No REM? Waste backs up.
That summation is only correct if the simplest answer is truly the correct one after all the alternatives have been examined.