While you sleep, your brain activates a remarkable cleaning system that helps remove waste products that build up during the day.
For centuries, scientists understood how the body removes waste through the lymphatic system.
But the brain seemed to be different.
Protected by a complex barrier system, it lacked the obvious waste-removal network found throughout the rest of the body.
So how does the brain clean itself?
The answer began to emerge in the late twentieth century when researchers discovered a previously unknown waste-clearance pathway within the brain.
Decades later, a landmark series of studies led by neuroscientist Maiken Nedergaard and colleagues revealed an even more surprising finding: this cleansing system becomes highly active during sleep.
Scientists named it the glymphatic system.
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What is the Glymphatic System?
Like the body’s lymphatic system, the glymphatic system helps remove waste.
But unlike traditional lymphatic vessels, it relies on glial cells, the brain’s support cells, to help move fluid through brain tissue.
The glymphatic system surrounds the brain’s blood vessels with fluid-filled spaces called perivascular spaces. Through these pathways, cerebrospinal fluid (CSF), the clear fluid that cushions the brain and spinal cord, circulates through the spaces between brain cells. As it moves, it helps carry away metabolic waste, excess proteins, and other cellular debris.
However, this cleaning system is not as active during the day.
Researchers discovered that the glymphatic system shifts into high gear during sleep, especially during deep non-rapid eye movement (NREM) sleep.
In early experiments, scientists injected tracer molecules into the brains of mice to track the movement of cerebrospinal fluid. When the animals were awake, fluid movement was limited. But when the mice entered NREM sleep, the flow of cerebrospinal fluid increased dramatically.
The spaces between brain cells also expanded by approximately 60%, creating more room for fluid to circulate and exchange waste products.
How does sleep create the ideal conditions for brain cleansing?
Scientists have identified several sleep-dependent mechanisms involved in brain cleansing.
First, the chemical environment of the brain changes during sleep. While awake, levels of the neurotransmitter noradrenaline are high, helping maintain alertness. During sleep, these levels decrease. This shift allows the spaces between brain cells to expand, making it easier for cerebrospinal fluid to flow through brain tissue.
Second, the rhythms of the body change during NREM sleep. Heart rate slows, breathing becomes deeper and more regular, and the coordination between these two processes increases.
These rhythmic changes create gentle mechanical pulses that influence blood vessel movement and help drive cerebrospinal fluid through the brain. These cleansing rhythms are significantly stronger during NREM sleep than during wakefulness.
Third, the brain’s own electrical activity appears to contribute to the process. During deep sleep, brain activity becomes slower and more synchronized, producing large waves of activity known as slow-wave activity (SWA).
Unlike the more scattered patterns of activity seen during wakefulness, these synchronized rhythms may help coordinate fluid movement across larger areas of the brain.
Together, these changes create the ideal conditions for the glymphatic system to perform cleansing.
Most of what scientists know about this process comes from animal studies, but researchers are beginning to find evidence that the same mechanisms may occur in humans.
In a groundbreaking human study using functional MRI technology, researchers observed increased cerebrospinal fluid movement as participants entered NREM sleep.
This increase was followed by slow-wave brain activity, increased blood flow, and a surge in cerebrospinal fluid movement.
This discovery suggests that deep sleep triggers a coordinated process that involves brain waves, blood flow, and fluid movement, all supporting the brain’s nightly cleaning process.
Why This Matters
Sleep is not a time when the brain shuts down but is an active biological process during which the brain performs essential maintenance.
The discovery of the glymphatic system has transformed scientists’ understanding of sleep, revealing that deep sleep may be one of the brain’s most important opportunities to remove waste and maintain its long-term health.
Excerpted and adapted from “The new science of sleep: From cells to large-scale societies” by Omer Sharon, et al. published in PLoS Biology, under the terms of the Creative Commons Attribution License.