Comment by SamBam

I feel this too, and always wondered if it related to the glymphatic system [1].

This is the system that clears out metabolic waste from the brain which builds up over time, and it's theorized that during slow-wave sleep in particular, the slow waves help pump out this waste fluid through microscopic channels the open up.

AFIAK, there were some researchers that were wondering if a drug of some kind could force this to happen more quickly, thus cutting down the amount we need to sleep. (Probably a bad idea.)

1. https://en.m.wikipedia.org/wiki/Glymphatic_system