Scientists at the University of Oslo have made discoveries about how and why “brainwashing”—a process of removing harmful waste products from the brain—happens when we are sleeping.

The brain is continually producing waste substances, and if too many accumulate, it raises the risk of Alzheimer’s disease and other neurodegenerative diseases, according to researchers in a study published in Nature Communications.

The “brainwashing” process is “far more efficient” when asleep than awake, though the reason is unclear, says associate professor Rune Enger, MD, PhD, at the Letten Centre at the University of Oslo and last author of the study, in a press release.

“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,” says Enger, who also works at the Institute of Basic Medical Sciences, in a press release. 

Blood Vessels in the Brain Dilate and Constrict in Certain Patterns While We Sleep

Waste products from the brain are cleared along specialized tunnels around brain blood vessels. Therefore movement of the blood vessels could affect this process. In a mouse model, researchers had the mice sleep naturally and then 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. Researchers say these movements probably pump fluids around the brain when we are asleep, cleansing the brain of waste substances.

Not Only Deep Sleep Is Important for Ridding the Brain of Waste 

Until now, it was believed that only deep sleep was involved in this cleansing of waste products. Yet in this study, 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 unique dance of brain arteries,” says one of the first authors, Laura Bojarskaite, PhD, in a press release.

Pumping of Blood Vessels 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 transporting 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 modeling 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,” says Kent-Andre Mardal, who led the computer modeling work in the study, in a press release.

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

Photo caption: Laura Bojarskaite and Rune Enger

Photo credit: Cecilie Bakken Hostmark