The blood vessels in the brain constrict and dilate in certain patterns when we sleep.

This is how your brain is washed while you sleep

The word 'brainwashing' usually triggers negative associations. But our brain health depends on it.

Researchers at the University of Oslo have recently made new and important discoveries about how and why this happens when we are sleeping.

The blood vessels in the brain constrict and dilate in certain patterns while we sleep and this is likely one of the key mechanisms driving the clearance of harmful waste substances from our brain.

“Our discoveries can help us find new ways to treat or even prevent Alzheimer’s disease and other neurodegenerative diseases. These findings can also help to create strategies to deliver drugs to the brain more efficiently” Associate Professor Rune Enger at the University of Oslo says.

Your brain is being washed while you sleep

'Brainwashing' or brain waste clearance is a process of removing harmful waste products from the brain.

Your brain is continually producing waste substances and if too many of them accumulate, it raises the risk of Alzheimer’s disease and other neurodegenerative diseases.

“The brainwashing process is far more efficient when you are asleep than when you are awake. The reason for this is not yet clear,” Enger says. He is the last author of the new article published in Nature Communications.

Blood vessels in your brain dilate and constrict

Waste products are cleared along specialised tunnels around the blood vessels in your brain. Movement of these blood vessels could therefore affect this process.

The researchers observed mice sleeping naturally, and examined what was going on in their brains using an advanced laser microscope.

They discovered that the blood vessels in the brain, specifically the arteries, dilated and constricted in certain patterns while the mice were sleeping. Such movements were not observed in awake mice.

The researchers believe that these movements pump fluids around our brain while we are asleep, cleansing it of waste substances.

Researchers Rune Enger and Laura Bojarskaite.

Deep sleep is not the only important factor

Up until now, it was believed that it was only deep sleep that was involved in this cleansing process.

In this study, however, the researchers observed something striking: The blood vessels in the brain constricted and dilated in patterns unique to each sleep stage of the entire sleep cycle, including deep sleep, REM sleep and even the brief awakenings that pepper our nightly sleep and are a natural part of a sleep cycle.

During deep sleep, the arteries slowly dilated and constricted, but as the mice transitioned to REM sleep these oscillations became smaller while the artery slowly dilated.

In REM sleep, arteries stayed dilated before quickly constricting at the end of a sleep cycle to the same size as before falling asleep. Such constrictions also happened during brief awakenings we experience while we sleep.

“It is as if every part of the sleep cycle has its own unique dance of brain arteries,” one of the first authors, Dr. Laura Bojarskaite says.

Boosts brain fluid flow and molecule transport

The researchers saw that these sleep cycle dependent artery dilations and constrictions affected the size of the channels around the blood vessels that are important for the transport of fluids and molecules in the brain.

These channels widened and narrowed in step with the blood vessels, leading Enger and his colleagues to believe that the flow of fluids was also affected.

The researchers then went on to use biomechanical computer modelling and simulations.

“To sum up, we found that the artery dilations and constrictions, and the simultaneous changes in the channels around them, had a big part to play in both the flow of fluids and the transport of substances in the brain,” Kent-Andre Mardal, who led the computer modelling work in the study, says.

The researchers believe that this new study may explain why the flow of fluids and waste clearance in the brain is different when you are asleep compared to when you are awake. It also identifies blood vessel dynamics in sleep as a potential target for the prevention of neurodegenerative disease and for improving drug delivery to the brain.

The research group

Researchers working at the GliaLab of the Letten Centre at the Institute of Basic Medical Sciences, the Faculty of Medicine, University of Oslo led the study in collaboration with researchers at the Institute of Mathematics, UiO, Simula and the Norwegian Computing Center (NR). The first authors of the study are Laura Bojarskaite, Alexandra Vallet and Daniel Marelius Bjørnstad.


Bojarskaite et al. Sleep cycle-dependent vascular dynamics in male mice and the predicted effects on perivascular cerebrospinal fluid flow and solute transport, Nature Communications, vol. 14, 2023. DOI: 10.1038/s41467-023-36643-5

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