Each time your head hits the pillow, it doesn’t take long for a wave of cerebrospinal fluid (CSF) to wash over your neurons — taking out the cellular junk with each rhythmic pulse.
The state of deep sleep possesses an ancient and restorative quality that helps to clear waste products from the brain. The recently discovered glymphatic system comprises a network of tunnels that facilitate the removal of metabolic byproducts. This cerebral apparatus plays a key role in brain health, and disturbances in its function are associated with neurodegenerative disease.
Logically, we often perceive sleep as a period of cognitive rest. Yet the keystone of the central nervous system remains surprisingly active during this time. Waste removal ramps up during sleep and remains comparatively low during wakefulness.
As the most energetically expensive organ in the body — consuming roughly 20% of the calories you consume each day — the brain churns out a lot of debris. Neurons rely on subunits of sugars, carbs, and proteins to transmit signals and form cellular components. They extract energy from these substrates through the process of cellular respiration. During these reactions, large quantities of toxic waste products are created. Among this barrage of cellular garbage, the brain excretes a quarter of an ounce of protein every single day.
Similar metabolic processes occur elsewhere in the body. Here, the lymphatic system plays the role of the cleanup crew. This body-wide network relies on human movement during wakefulness to carry worn-out proteins from the space between cells into the bloodstream for elimination. Despite centuries of study, anatomists failed to identify the existence of these lymph-carrying vessels within the brain and nervous system — leading scientists to believe the brain simply dealt with waste independently.
A groundbreaking scientific finding rattled this paradigm when researchers discovered a unique network of vessels running parallel to the vasculature within the brain. A publication within the journal Science Translational Medicine documents how a research team discovered the channels by tagging proteins with a fluorescent dye. As they tracked the glowing molecules, they witnessed waves of CSF flow through the interstitial space of the brain and wash them into these newfound drainage pathways.
Unlike the rest of the human body, the brain exists in a compact space within the skull. Amongst a crowded chamber of grey and white matter and the blood-pumping circulatory network, little room remains for a set of stand-alone lymphatic vessels. Instead, the brain developed specialized anatomy to deal with this issue — it built a convenient plumbing system that encases arteries and veins.
Blood vessels carry vital oxygen and nutrients to every single cell within the brain, granting the glymphatic system easy access to every information-carrying neuron in our heads. Described by research team member Maiken Nedergaard as the “garbage truck of the brain” and the “brain drain”, these structures fill in the gaps in neuroanatomy and explain why scientists hitherto failed to identify the expansion of lymph vessels in the central nervous system.
Perched among our neurons, glial cells make up a large portion of the human brain. Tasked with maintaining homeostasis and playing a supporting role for other cells, these biological subunits are the building blocks to the glymphatic system. The very word “glymphatic” stems from “glial” and “lymphatic”, referring to a lymphatic-like system composed of glial cells.
Astrocytes — a specific type of glial cell — possess broad curved extensions known to science as endfeet. These structures reach out to arteries, capillaries, and veins, covering their exterior and creating a hollow channel between the two types of tissue. These doughnut-shaped tunnels serve to transport fresh CSF into the brain and drain protein-laden liquid away from the organ.
We spend one-third of our entire lives asleep. The majority of animal species enter this state of consciousness daily, despite placing themselves at great risk to predators. The discovery of the glymphatic system helps to explain why evolution makes this exceptional tradeoff.
Every night, adult humans undergo 4-5 cycles of sleep within the recommended 8 hours. Each cycle consists of light sleep, deep sleep, and dream-inducing REM sleep. When we fall short of meeting our bodies needs, we build up a sleep debt and experience deficits in learning, memory, and concentration — factors paramount to survival in nature.
During wakefulness, the brain rationalizes, thinks, creates, and coordinates movements — a state associated with high-frequency alpha brain waves. During deep sleep, the brain instead starts to operate using slow frequency delta waves, giving the electrochemical organ a chance to divert resources towards equally vital physiological processes.
Research conducted at Boston University during 2019 captured images that display the ebbs and flows of CSFthrough the glymphatic system and brain during deep sleep. As the brain descends into slow-wave activity, blood starts to flow out of the brain. This initiates a shift in pressure that causes a watery surge of CSF.
This fresh fluid enters the brain through the glymphatic pipes that surround arteries — vasculature that carries blood away from the heart. From here, it washes across the space between brain cells, picking up soluble waste protein as it goes. Eventually, blood flows back into the brain, driving the CSF into the glymphatic system that surrounds the veins.
From here, the worn-out proteins enter the blood, are transported into the lymphatic system, back into the main circulatory pathways of the body, and processed for elimination by the liver and kidneys. Far from a chaotic splashing of bodily fluids, the CSF and blood exchange positions in the brain in a rhythmic and oscillatory dance throughout deep sleep phases.
This nightly detoxification process plays a critical role in maintaining cognitive health. The action of the glymphatic system helps to purge proteins called beta-amyloid and tau, molecules that play notorious roles in the onset and progression of Alzheimer’s disease. Our bodies constantly manufacture these proteins, but problems in clearance lay the foundation for the pathology.
In the Alzheimer’s brain, large quantities of amyloid-beta begin to clump together, form plaques, and disrupt neuronal function. Tau also begins to form problematic tangles that damage neurons and their communication systems.
Researchers previously thought the brain disposed of and recycled these damaging molecules in isolation. However, the discovery of the glymphatic system unveiled that the slow-wave state of deep sleep flushes these neurodegenerative proteins out of the brain entirely.
Interestingly, many patients begin to report a decrease in sleep quality before the onset of Alzheimer’s. The disease also becomes more prevalent with age, a biological process associated with lighter sleep. The mechanism of glymphatic clearance has fuelled a scientific interest in manipulating this system using sleep therapy and other strategies to induce deep cleansing sleep in at-risk populations.
These incredible scientific insights into how the body cleans one of our most essential organs send a powerful message. We need sleep to function, survive, and thrive. Humans have drifted a long way from our evolutionary course. From a world of natural light cycles, we find ourselves peering into the digital world late into the night, an act that suppresses key sleep hormones. We experience levels of chronic stress from work, crowds, and media bombardment — factors absent from our ancestral home that deprive us of vital sleep.
Modern humans are also chronically stationary and deprived of optimal physical stresses that cause beneficial adaptations in our muscles, tendons, bones, and beyond. Exercise helps to provide the movement our species requires to remain healthy. Aside from musculoskeletal benefits, early animal research suggests exercise may double glymphatic activity two-fold over a five-week period.
The brain operates optimally in a highly controlled environment, regulated in part by the glymphatic system. A few nights of subpar sleep can impede cognitive function, whereas years of incomplete sleeping cycles may give rise to insidious chronic disease.
The bottom line: deep sleep literally cleans the brain. Aside from novel health optimization strategies and extreme lifestyle overhauls, a full night’s sleep remains a foundational pillar of human wellbeing.
Luke Sumpter is a freelance journalist that specializes in health, wellness, and alternative therapies. Currently, he’s working on a dissertation exploring the emerging role of the endocannabinoid system in orthopaedic medicine.