Cleaning the brain: Neurons flush out waste while we sleep

Despite its appearance of tranquility, sleep is a time of significant activity for the brain, not in silence but in a state of dynamic electrical activity. 

As the world quiets down, the brain remains a hub of activity, with neurons firing off bursts of electrical pulses that coalesce into rhythmic waves, indicative of intense neural function. The question arises: Why does the brain stay so active during our rest?

Synchronized neural activity

Scientists at Washington University School of Medicine in St. Louis have discovered that slow brain waves play a crucial role in cleaning the brain during sleep. Nerve cells synchronize to produce these waves, which in turn propel fluid through the brain’s complex tissue, effectively cleaning it.

“These neurons are miniature pumps. Synchronized neural activity powers fluid flow and removal of debris from the brain,” said Li-Feng Jiang-Xie, a postdoctoral research associate at WUSTL. 

“If we can build on this process, there is the possibility of delaying or even preventing neurological diseases, including Alzheimer’s and Parkinson’s disease, in which excess waste – such as metabolic waste and junk proteins – accumulate in the brain and lead to neurodegeneration.”

Metabolic waste 

Brain cells are responsible for a wide range of functions, including thought, emotion, movement, memory, and problem-solving. These processes are energy-intensive, leading to the production of metabolic waste. 

“It is critical that the brain disposes of metabolic waste that can build up and contribute to neurodegenerative diseases,” said senior author Jonathan Kipnis, a professor of pathology and immunology at the same university. 

“We knew that sleep is a time when the brain initiates a cleaning process to flush out waste and toxins it accumulates during wakefulness. But we didn’t know how that happens. These findings might be able to point us toward strategies and potential therapies to speed up the removal of damaging waste and to remove it before it can lead to dire consequences.”

Fluid movement and rhythmic waves

According to the experts, cleaning the dense structure of the brain is a complex task. Cerebrospinal fluid must navigate through the brain, collecting waste before exiting and filtering through the body’s lymphatic system. The researchers demonstrated that neurons’ rhythmic waves are essential for driving this fluid movement.

By specifically silencing regions in the brains of sleeping mice, preventing neurons from generating these waves, the researchers observed that cerebrospinal fluid could not circulate properly, trapping waste within the brain tissue.

Cleansing the brain

“One of the reasons that we sleep is to cleanse the brain,” Kipnis said. “And if we can enhance this cleansing process, perhaps it’s possible to sleep less and remain healthy. Not everyone has the benefit of eight hours of sleep each night, and loss of sleep has an impact on health.” 

“Other studies have shown that mice that are genetically wired to sleep less have healthy brains. Could it be because they clean waste from their brains more efficiently? Could we help people living with insomnia by enhancing their brain’s cleaning abilities so they can get by on less sleep?”

Study implications 

The researchers also found that the amplitude and rhythm of brain waves vary throughout sleep, affecting the force with which fluid moves. They now aim to understand the implications of these variations and identify which brain regions are most at risk of waste accumulation.

“We think the brain-cleaning process is similar to washing dishes. You start, for example, with a large, slow, rhythmic wiping motion to clean soluble wastes splattered across the plate. Then you decrease the range of the motion and increase the speed of these movements to remove particularly sticky food waste on the plate,” said Jiang-Xie.

“Despite the varying amplitude and rhythm of your hand movements, the overarching objective remains consistent: to remove different types of waste from dishes. Maybe the brain adjusts its cleaning method depending on the type and amount of waste.”

The study is published in the journal Nature.

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