Did you know that you get your brain cleansed every time you go deep into slumber?

A team of neuroscientists from Boston University has found that during sleep, blood flows out from your brain, then waves of watery liquid called cerebrospinal fluid (CSF) flows in pulsing rhythmically to wash your brain. The action also flushes out any toxic, memory-impairing proteins that shouldn’t be in your head.

The study, published in the journal Science, is the first to illustrate cerebrospinal fluid in action and its association with brain wave activity and blood flow. Researchers say their findings could also provide new insights about a variety of neurological and psychological disorders, especially the ones associated with disrupted sleep patterns, including autism and Alzheimer’s disease.

“We’ve known for a while that there are these electrical waves of activity in the neurons,” said the study coauthor Laura Lewis of BU. “But before now, we didn’t realize that there are actually waves in the CSF, too.” 

As you get older, the brain generates fewer slow waves. This could decrease the blood flow in the brain and reduce the pulsing of CSF during sleep, leading to increased buildup of toxic proteins and a decline in memory abilities.

Earlier studies have explored CSF flow and slow-wave activity in helping rinse out toxins from the brain, but this pulsing action had never been charted until now.

So combining brain waves with the flow of blood and CSF, this new study could help jump-start research into normal age-related impairments as well. Another fascinating yield of the study is that it’s now possible to tell if a person is sleeping or not simply by examining the CSF activity on a brain scan.

Nocturnal pulsing process in action. During sleep, the brain exhibits waves of blood oxygenation (red) followed by waves of cerebrospinal fluid (blue).

“It’s such a dramatic effect,” she explained. “CSF pulsing during sleep was something we didn’t know happened at all, and now we can just glance at one brain region and immediately have a readout of the brain state someone’s in.”

For the current study, the team recruited 13 participants between the ages of 23 and 33, and monitored their brain waves while dozing off inside an MRI machine. In the future, they would be looking at older adults to further examine how flow of blood and CSF in the brain slacken with age. Another improvement would be to come up with a more sleep-conducive method of imaging CSF, because the participants in this study were tasked to sleep inside an extremely noisy MRI machine.

One explanation as to how exactly brain waves, blood flow and CSF stay so perfectly in sync: when neurons shuts off, they don’t need as much oxygen, so the blood moves away. And as the blood leaves, CSF quickly flows in to maintain pressure in the brain.  

“That’s just one possibility,” said Lewis. “What are the causal links? Is one of these processes causing the others? Or is there some hidden force that is driving all of them?”