What happens to your brain when you sleep

A recent study suggests that amazing things happen with a person's brain while sleeping.

The neurons will go quiet and a few seconds later, blood flows out of the head. Then, a watery liquid called cerebrospinal fluid (CSF) flows in, washing through the brain in rhythmic, pulsing waves.

The study - published in Science - is the first to illustrate that the brain's CSF pulses during sleep, and that these motions are closely tied with brain wave activity and blood flow.

"We've known for a while that there are these electrical waves of activity in the neurons," said study co-author Laura Lewis, a BU College of Engineering assistant professor of biomedical engineering and a Center for Systems Neuroscience faculty member.

"But before now, we didn't realise that there are actually waves in the CSF, too," added Lewis.

This research may also be the first-ever study to take images of CSF during sleep.

Lewis hopes that it will one day lead to insights about a variety of neurological and psychological disorders that are frequently associated with disrupted sleep patterns, including autism and Alzheimer's disease.

The coupling of brain waves with the flow of blood and CSF could provide insights about normal age-related impairments as well.

Earlier studies have suggested that CSF flow and slow-wave activity both help flush toxic, memory-impairing proteins from the brain. 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.

Although researchers have tended to evaluate these processes separately, it now appears that they are very closely linked.

To further explore how ageing might affect sleep's flow of blood and CSF in the brain, Lewis and her team plan to recruit older adults for their next study, as the 13 subjects in the current study were all between the ages of 23 and 33.

Lewis said they also hope to come up with a more sleep-conducive method of imaging CSF.

Wearing EEG caps to measure their brain waves, these initial 13 subjects were tasked with dozing off inside an extremely noisy MRI machine, which, as anyone who has had an MRI can imagine, is no easy feat.

One of the most fascinating yields of this research, Lewis said, was that they can tell if a person is sleeping simply by examining a little bit of CSF on a brain scan.

"It's such a dramatic effect," she said. "[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.