Non-REM.

> By studying 11 people as they slept, Lewis and her team identified three parts to the rinse cycle. First, there is electrical activity that’s definitive of slow wave sleep. Then neurons “go quiet” as Lewis puts it, blood volume within the brain decreases, and cerebral spinal fluid flows upwards. This flow happens about every 20 seconds during slow wave sleep, according to the study.

https://www.inverse.com/article/60583-sleep-rinse-cycle-alzh...