As we fall asleep, the brain begins clearing out waste.
It operates like a late-night laundry service, with all the water valves opened and washing machines running at full capacity to remove dirt from piles of clothes, flushing the wastewater into the drain.
The brain continuously produces various wastes, and if these are not cleared in time, we feel it. The signs can range from feeling foggy and fatigued to experiencing cognitive impairment.
Fortunately, efforts can be made to optimize waste clearance during the night.
Flush the Waste
The human brain is one of the most metabolically active organs, accounting for about 20 percent of the body’s total energy expenditure. This high level of activity generates significant waste. Smaller byproducts, such as carbon dioxide, urea, and ammonia, diffuse into capillaries and are cleared through the bloodstream. Larger neurotoxic proteins—including beta-amyloid and tau, both widely associated with an increased risk of Alzheimer’s disease, cannot be eliminated through the bloodstream alone due to their size.In the past, it was believed that the brain lacked a lymphatic system to remove waste and relied solely on internal mechanisms for clearance.
However, in 2012, researchers discovered a specialized mechanism within the brain, analogous to the lymphatic system and capable of flushing out larger waste products from deep within the brain. This system was named the glymphatic system, a portmanteau of “glial” (referring to glial cells) and “lymphatic.” It is also known as the pseudo-lymphatic system.
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How the brain removes waste through the glymphatic system. Illustration by The Epoch Times
Surrounding the arteries in the brain is a sheath-like structure, with cerebrospinal fluid flowing through the space between the artery and this sheath. During sleep, the brain’s blood vessels constrict, increasing the space between the vessels and the sheath, which allows more cerebrospinal fluid to flow in. As the arteries pulse, the cerebrospinal fluid is pumped through brain tissue, flushing out waste—such as beta-amyloid and tau proteins—from the deeper spaces between brain cells, eventually clearing it from the brain.
Deep Sleep
“Waste-removing processes in the brain barely operate during wakefulness. It is very much a process that occurs in our deep-sleep stages,” Moira Junge, who holds a doctorate in health psychology and is the CEO of the Sleep Health Foundation in Australia and an adjunct clinical associate professor at Monash University, told The Epoch Times.Sleep is divided into two states: rapid eye movement (REM) sleep and nonrapid eye movement (NREM) sleep. NREM makes up 75 percent of total sleep time and is further divided into three stages, N1, N2, and N3—each reflecting progressively deeper levels of sleep.
During N3, brainwaves are at their slowest.
“It’s such a deep sleep that you’re not easily disturbed by the external environment; for example, you don’t hear the dog barking outside nor hear your partner come to bed,” Junge said.
During sleep, the body moves through the stages sequentially, forming a complete sleep cycle lasting around 90 minutes. Throughout the night, a person typically experiences four to five sleep cycles.
The stages of sleep. Illustration by The Epoch Times
The glymphatic system becomes more active during sleep, especially during deep sleep, allowing for more effective waste clearance, said psychiatrist Dr. Jingduan Yang, founder of the Yang Institute of Integrative Medicine in Pennsylvania.
In a mouse study published in Science, researchers used tracers to monitor changes in cerebrospinal fluid flow. They found that during sleep, the interstitial, or intervening, space expanded by more than 60 percent, and the tracer influx increased. The brain’s clearance rate of beta-amyloid doubled during sleep (or under anesthesia) compared to the awake state.
Accumulated Beta-Amyloid
Unfortunately, Americans today are sleeping less than ever.In 2023, 42 percent of Americans perceive that they get enough sleep, according to Gallup’s December 2023 poll. One in five people sleep fewer than five hours a night—compared to just 3 percent in 1942.
Shorter sleep duration can also be attributed to people going to bed increasingly later. One study found that delaying bedtime by just one hour reduces total sleep by 14 to 33 minutes each night.
In addition to going to bed later and sleeping less, we are also not sleeping well. According to the American Psychiatric Association, more than 50 million people in the United States suffer from chronic sleep disorders like insomnia and sleep apnea.
These issues directly reduce and disrupt deep sleep, shortening the critical window during which the glymphatic system works at peak efficiency. This, in turn, leads to greater waste accumulation in the brain.
People reporting less adequate sleep and more sleep problems had greater amyloid burden in Alzheimer’s disease-sensitive brain regions.
A 2021 human study found that even a single night of sleep deprivation can impair the brain’s ability to clear waste.
An earlier clinical trial showed that despite the expected relatively sizeable interindividual variation in levels of a type of amyloid-beta, the average beta-amyloid accumulation from three morning samples of unrestricted sleep was 6 percent lower than that of three evening samples.
In comparison, participants who stayed awake for 24 hours exhibited amyloid-beta levels up to 75.8 picograms per milliliter higher. This demonstrated that unrestricted sleep reduced amyloid-beta proteins but that sleep deprivation counteracted this effect. Furthermore, the longer the sleep duration—provided it was not excessive—the greater the reduction in beta-amyloid biomarkers.
Symptoms of Toxicity
The accumulation of waste in the brain can lead to various symptoms.For instance, if this waste is not cleared and continues to accumulate, it can become difficult to stay clear-headed, Junge explained.
“The most common symptom is a decline in cognitive function,” including memory loss, difficulty concentrating, and trouble managing complex tasks, Yang told The Epoch Times.
The long-term accumulation of these waste products can also affect mood, leading to anxiety, depression, or irritability. Yang further explained that this accumulation may be directly linked to neurodegenerative diseases, such as Alzheimer’s or Parkinson’s, as both conditions are closely associated with the buildup of beta-amyloid and tau proteins in the brain.
A 2021 longitudinal study with an average follow-up of 25 years and involving 7,959 older adults found that those who consistently slept less than six hours per night had a 30 percent higher risk of developing dementia compared to those who slept seven hours.
A 2019 study tracking more than 13,000 Dutch older adults over an average of eight years showed that declines in sleep quality and reduction in sleep duration increased the risk of developing Parkinson’s disease within the next six years by 76 percent and 72 percent, respectively.
Optimize Glymphatic Function
An interesting fact: Deep sleep lasts longer during the first few sleep cycles (the first half of the night) and gradually shortens—or may not occur at all—in later cycles.This subtle shift reflects, to some extent, the brain’s prioritization of the cleaning and repair processes.
“The glymphatic system likely works more in the first half of sleep because this period includes longer periods of deep sleep (N3), where the system is most efficient,” Kiminobu Sugaya, professor of medicine at the University of Central Florida College of Medicine and head of the neuroscience division at the Burnett School of Biomedical Sciences, told The Epoch Times. “As the night progresses and the duration of deep sleep decreases, the system may still function but likely with less efficiency.
“To optimize brain waste clearance via the glymphatic system, it is important to align sleep with the body’s natural circadian rhythm,” he said, emphasizing that going to bed earlier promotes deep sleep during the early part of the night.
Research has demonstrated the significant impact of circadian rhythm on the glymphatic system and cerebrospinal fluid distribution.
“The ideal bedtime should align with the body’s circadian rhythm, typically between 10 and 11 p.m.,” Yang said, echoing Sugaya’s view.
However, he also said that the pace of modern life makes it challenging for many people to go to bed early. That said, he strongly advises not going to bed later than midnight, as it may impair the brain’s repair functions.
Jonathan Liu, a registered traditional Chinese medicine (TCM) practitioner in Canada and a lecturer of TCM at Ontario Public College, explained that, according to TCM theory, the period between 9 p.m. and 11 p.m. corresponds to the body’s circulation of energy. Sleeping during this time can allow the energy to rebalance. Melatonin production also increases during this time.
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However, Junge explained that some people have different sleep patterns. Their biological clocks may be delayed, meaning their melatonin production and sleep onset occur later than those of early risers, which is why they tend to stay up late and wake up later. Even if these people go to bed after midnight, getting up after 9 a.m. can still provide enough rest.
Side Sleeping
Sleep posture directly influences the brain’s waste clearance.The glymphatic system operates more efficiently in the side-lying position than sleeping on the back or stomach. Prone (stomach) sleeping impairs cerebral blood flow and increases sympathetic nervous activity, triggering the release of stress hormones that suppress glymphatic function. In contrast, side sleeping reduces sympathetic tone, possibly improving glymphatic influx.
Some experts suggest sleeping on the right side is more beneficial than sleeping on the left. With the heart positioned higher, blood circulation improves, and venous return increases, allowing the heart to work more efficiently while keeping sympathetic nervous activity low.
A 2019 study found that patients with neurodegenerative diseases—such as mild cognitive impairment, Alzheimer’s disease, Lewy body dementia, and Parkinson’s disease—were significantly more likely to spend over two hours per night sleeping on their backs. The researchers speculated that gravity influences the movement and distribution of blood leaving the brain, suggesting that head position may affect the brain’s ability to clear proteins efficiently.
Exercise, Breathing, and Meditation
The heart’s ability to pump blood and the pulsation of brain arteries are closely tied to the efficiency of the glymphatic system. The more blood the heart pumps with each beat and the stronger the arterial pulsations, the more cerebrospinal fluid flows into brain tissue to flush out waste.However, certain cardiovascular conditions can impair these functions. For example, hypertension has been shown to hinder glymphatic function. Although high blood pressure does not change artery diameter, it alters arterial wall pulsations, affecting the flow of cerebrospinal fluid.
Physical exercise enhances cardiopulmonary function and helps regulate blood pressure.
Using real-time MRI, researchers later discovered that breathing significantly influences the efficiency of cerebrospinal fluid (CSF) flow.
During sleep, breathing drives the flow of CSF through the glymphatic system. Shallow breathing reduces pressure in the chest, indirectly decreasing pressure in the skull. Increasing lung capacity through regular exercise and practicing deep-breathing techniques can enhance breathing during sleep. Along with age-related changes in arterial pulsatility in the brain, these pressure changes inhibit glymphatic function.
Another effective way to enhance glymphatic function and support brain health is through meditation.
“Meditation has a good body of evidence that it can help with sleep,” Junge said. She explained that it helps reduce mental hyperactivity and excessive tension, allowing people to relax fully.
A 2022 study found that mindfulness-based meditation significantly improved both the duration and quality of sleep in 106 older adults with insomnia.
Stress triggers the release of norepinephrine, a hormone that increases alertness. It inhibits CSF secretion and narrows the space between brain cells, restricting CSF flow, which impairs glymphatic function.
A prospective randomized trial found that after 14 weeks of meditation practice, norepinephrine levels in the blood of congestive heart failure patients decreased by about 43 percent.
Meditation has also been shown to increase blood flow to the brain. A 2022 study found that after eight weeks of a mindfulness-based stress reduction program, older adults with an average age of 79 experienced significant increases in cerebral blood flow across multiple brain regions, even at rest. Their brains also exhibited positive attention and memory functional changes.
“The brain consumes a lot of energy, and during sleep, such energy and metabolism goes down,“ said Sugaya, ”so then we can put more of that energy to this glymphatic system.”
He explained that this is why, during the night, we have a better “washout.” He also noted that during sleep, because the neurons are not that busy, the space between them expands. These factors work together perfectly to clear waste.
People are used to putting work, entertainment, or fitness goals ahead of getting adequate sleep, Junge said.
“It’s not too late to instigate making sleep more of a priority.”