Here's how sleep helps to clear the brain

Boston University researchers reveal what really happens within your brain while you sleep. A new study reveals cerebrospinal fluid (CSF), a watery liquid, flows into your brain and washes out the toxic and memory-impairing proteins.

This is the first study to show how the brain's CSF pulses have a strong link with blood flow and brain wave activity. "We've known for a while that there are these electrical waves of activity in the neurons," study co-author Laura Lewis, a BU College of Engineering assistant professor of biomedical engineering and a Center for Systems Neuroscience faculty member, told a news portal. Adding, "But before now, we didn't realize that there are actually waves in the CSF, too."

Researchers hope the findings shed better light on a range of neurological and psychological disorders that are linked to sleep, autism and Alzheimer's disease. The team also believe this combination of blood flow, CSF and brain waves can give better insight about age-related issues. "As people age, their brains often generate fewer slow waves. In turn, this could affect the blood flow in the brain and reduce the pulsing of CSF during sleep, leading to a buildup of toxic proteins and a decline in memory abilities," authors of the study revealed.

For the study, the team examined 13 participants between the ages of 23 and 33. All the participants wore EEG caps so the team could measure their brain waves. "We have so many people who are really excited to participate because they want to get paid to sleep," Lewis told a news portal. Adding, "But it turns out that their job is actually -- secretly -- almost the hardest part of our study. We have all this fancy equipment and complicated technologies, and often a big problem is that people can't fall asleep because they're in a really loud metal tube, and it's just a weird environment."

The team was also fascinated to learn that they were able to identify if a person is sleeping by taking a quick look at the CSF on a brain scan. "It's such a dramatic effect," Lewis told a news portal. Adding, "[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."

Now, researchers want to understand how the brain waves, blood flow, and CSF work together. "We do see that the neural change always seems to happen first, and then it's followed by a flow of blood out of the head, and then a wave of CSF into the head," Lewis told a news portal.

One reason could be that neurons don't require a lot of oxygen when they switch off and so the blood can leave the area and allow the CSF to flow in. "But that's just one possibility," Lewis told a news portal. Adding, "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?"

The study's findings were originally published in the journal Science.

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