I'm super curious as well, I've been digging into glutamate inhibition on neuronal NMDA receptors and found this while looking into cerebral spinal fluid flushes during REM sleep.

https://academic.oup.com/sleep/article/37/12/1919/2416791

"These results demonstrate that sleep deprivation and subsequent recovery-induced changes in high-energy phosphates primarily occur in gray matter, and increases in phosphocreatine after recovery sleep may be related to sleep homeostasis."

It's not an answer, in fact it just brings more questions. Human sleep and the brain is a tough puzzle.