The Brain’s Hidden Cleaning System: Robert Sapolsky on the Glymphatic System and Mental Illness

For decades, neuroscientists understood the brain as an organ of electrical signaling, neurotransmitters, and vast neural networks. But one fundamental mystery lingered in the background: how does the brain clean itself?

Unlike the rest of the body, the brain seemed strangely disconnected from the lymphatic system—the network responsible for clearing waste and toxins from tissues. For years, researchers largely assumed the brain simply handled waste removal in some diffuse, poorly understood way.

Then, in 2012, a breakthrough changed the field.

Danish-American neuroscientist Maiken Nedergaard and her colleagues at the University of Rochester identified what is now called the glymphatic system, a previously unknown cleaning mechanism that flushes toxic debris out of the brain. The discovery immediately transformed scientific understanding of sleep, neurodegeneration, inflammation, and possibly even psychiatric illness.

According to Robert Sapolsky, the glymphatic system is one of the most fascinating recent discoveries in neuroscience because it reveals that the brain undergoes a kind of nightly cleansing cycle—essentially washing itself while we sleep.

At the center of this process are glial cells, the often-overlooked support cells of the nervous system. For much of modern neuroscience, neurons received nearly all the attention, while glial cells were treated as passive scaffolding. The glymphatic system changed that perception dramatically.

The mechanism itself is remarkably elegant.

Around arteries in the brain lie fluid-filled channels called perivascular spaces, also associated with structures known as Virchow–Robin spaces. Specialized glial cells surrounding these arteries use protein channels called aquaporins to pump water into these spaces. Then, with each heartbeat, pulsations of blood push this fluid through the brain in slow cleansing waves.

These waves carry away metabolic waste, inflammatory molecules, and potentially toxic proteins before draining into the body’s lymphatic system.

Sapolsky compares the process to a giant wave pool. The glial cells act like pumps generating water movement, while rhythmic pulses send waves sweeping across the brain, carrying debris toward drainage points. Instead of cleaning dirty pool water, however, the glymphatic system clears neurological waste that could otherwise accumulate and damage brain tissue.

One of the most important features of this system is that it becomes dramatically more active during sleep—especially during deep slow-wave sleep.

During these stages, the spaces between brain cells expand, allowing cleansing fluid to move more freely through neural tissue. This means sleep is not simply a period of rest or reduced activity. It is also a period of biological maintenance during which the brain actively clears potentially harmful substances.

Among the substances removed are beta-amyloid and tau proteins, the two molecules most strongly associated with Alzheimer’s disease. Studies have shown that sleep deprivation reduces the brain’s ability to clear these proteins efficiently, suggesting that chronic poor sleep may increase long-term risk for neurodegeneration.

This connection between sleep and brain health has become one of the most important developments in modern neuroscience. It reframes sleep not as a luxury or passive state, but as an essential biological requirement for neural survival.

But the implications may extend beyond neurological diseases.

Researchers are increasingly exploring whether abnormalities in the glymphatic system might also contribute to psychiatric disorders such as depression and schizophrenia.

Using advanced imaging methods like diffusion tensor imaging, scientists can indirectly estimate glymphatic activity in living humans. Early findings suggest that people with major depressive disorder often show impaired glymphatic function. These abnormalities are associated with greater cognitive impairment, psychomotor slowing, fatigue, and elevated inflammation.

This is particularly significant because inflammation itself is increasingly implicated in psychiatric illness. If the glymphatic system helps remove inflammatory molecules from the brain, then impaired cleansing could theoretically contribute to chronic inflammatory states associated with mood disorders.

Similar findings have emerged in schizophrenia research. Some studies indicate that glymphatic abnormalities are present even in individuals experiencing their first psychotic episode before long-term medication exposure. That observation raises the possibility that dysfunction in the system may precede at least some psychiatric symptoms rather than merely resulting from them.

Yet the direction of causality remains uncertain.

It is possible that impaired glymphatic clearance contributes to the buildup of inflammatory or neurotoxic substances that increase vulnerability to psychiatric disorders. But it is equally plausible that psychiatric illness itself disrupts sleep, stress regulation, metabolism, and neural activity in ways that gradually damage glymphatic function.

In other words, the relationship may be bidirectional.

The longer individuals live with severe psychiatric disorders, the more pronounced glymphatic abnormalities often appear to become. This suggests that mental illness may both influence and be influenced by disruptions in the brain’s cleaning system.

Genetics may also play a role.

Some researchers have begun investigating variants of the aquaporin genes responsible for the water channels that drive glymphatic flow. Early evidence suggests that certain aquaporin gene variants may occur more frequently in subsets of people with schizophrenia, though the findings remain preliminary.

Despite the excitement surrounding these discoveries, Sapolsky emphasizes caution. Psychiatric disorders are extraordinarily complex conditions shaped by hundreds of genes, environmental influences, developmental experiences, inflammation, stress biology, and social factors. Even if the glymphatic system proves important, it is unlikely to provide a single explanation or universal treatment target.

Still, the discovery of the glymphatic system has fundamentally altered scientific understanding of the sleeping brain.

For generations, sleep appeared mysterious because consciousness simply shut off for hours at a time. Now neuroscience increasingly suggests that while the conscious mind rests, the brain itself becomes intensely active in repair, maintenance, and detoxification.

The brain, it turns out, may spend each night quietly cleaning the consequences of being awake.

© 2000-2030 Sieglinde W. Alexander. All writings by Sieglinde W. Alexander have a five-year copyright. Library of Congress Card Number: LCN 00-192742 ISBN: 0-9703195-0-9 

Glymphatic System by Robert Sapolsky https://www.youtube.com/watch?v=IPts9X3XNBI