Cleaning our brains during deep sleep

I monitor my sleep both with the Oura Ring and the Apple watch, and have observed that my mental clarity and feeling rested on waking positively correlates with the duration of my non-REM deep sleep. If it is 30 minutes or more I feel perky and rested, if 15 minutes or less, not so much. 

This made me note the review article in Brain magazine by by Hauglund and Nedergaard titled "Is glymphatic clearance the secret to restorative sleep?" that I came across during my TOC (tables of contents) scanning of Neuroscience journals.  I pass on their abstract below ( (followed by excerpts from google search's response to my typing "Is there a correlation between deep sleep and brain glymphatic clearance?" in the URL line of my Firefox browser...First google's text, and then a bouncy YouTube video it points to:

Abstract

Despite the universal need for sleep across animal species, the biological mechanisms underlying the restorative aspects of sleep remain poorly understood. While sleep architecture is traditionally evaluated using EEG, multiple studies have shown a mismatch between EEG-defined parameters and subjective sleep quality. In particular, slow-wave activity—a hallmark of non-REM (NREM) sleep—does not consistently align with perceptions of sleep depth or subsequent well-being. This discrepancy suggests that core physiological processes beyond neuronal activity contribute to the restorative value of sleep.

Recent discoveries have identified the glymphatic system as a brain-wide clearance pathway that facilitates the removal of metabolic waste during sleep. In rodents, glymphatic activity is driven by a complex interplay between norepinephrine oscillations, vascular dynamics and CSF flow—particularly during NREM sleep. Human imaging studies have revealed parallel signatures, including large-scale CSF pulsations and inverse coupling between blood and CSF volumes during sleep. Disruption of these infraslow dynamics has been observed in conditions such as insomnia, chronic fatigue, and sleep misperception, suggesting a potential link between impaired glymphatic function and non-restorative sleep.

This review synthesizes the current evidence for glymphatic clearance as a contributor to sleep’s restorative function, discusses emerging biomarkers, such as cyclic alternating patterns and pupil-based proxies of noradrenergic tone, and highlights the need for improved methods to evaluate glymphatic function in humans. We propose that brain clearance may represent a key physiological determinant of restorative sleep and suggest future directions to test this hypothesis across health and disease.

Google search's response to my typing "Is there a correlation between deep sleep and brain glymphatic clearance?" (drawn largely from Cleveland Clinic's Glymphatic System Overview)

Yes, there is a widely recognized correlation between deep sleep and brain glymphatic clearance. The brain's glymphatic system—a network that flushes out metabolic waste, including toxic proteins associated with neurodegenerative diseases like Alzheimer's—is highly active during sleep, particularly during slow-wave deep sleep. 

The correlation between this clearance mechanism and deep sleep is based on several biological factors: 

Cellular Shrinkage: During non-REM (NREM) slow-wave sleep, brain cells subtly shrink, expanding the space between them by up to 60%. This expansion allows cerebrospinal fluid (CSF) to flow more freely and wash away waste.

Lower Norepinephrine Levels: The production of the neurotransmitter norepinephrine naturally declines during sleep. This reduction relaxes the glymphatic vessels and decreases resistance, allowing for significantly more efficient fluid exchange.

Active Waste Removal: During this restorative phase, the glymphatic system synchronizes with brain waves to efficiently clear out metabolic byproducts like amyloid-beta and tau, which accumulate in the brain during wakefulness.