Another piece of the puzzle explaining sleep’s effect on our mental health and brain functioning has been uncovered in new research published this week in the journal Science!

In an ironic case of self-sacrifice, Laura Lewis and her team at Boston University spent many late nights in the lab analyzing the brains of sleeping research participants. Their aim was to uncover the role of sleep in the detoxification of the brain. Knowing whether and how sleep helps detoxify the brain is key to cracking the elusive scientific question of why we need sleep.

The research team’s sleep-deprived nights provided data that reveals a previously unconfirmed link between brain activity during sleep and the activity of cerebrospinal fluid, which helps to clear metabolic junk from the brain.

It’s well established that sleep quantity and quality has a major impact on your health and wellbeing. Sleep is important for both high-level cognition and for general brain maintenance as you age. But we don’t fully understand exactly how sleep benefits the brain.

To test the role of sleep on brain health, Laura Lewis’s team brought research participants into their Boston University lab late at night and asked them to sleep in an MRI machine. After setting up camp in the machine around midnight, participants had their brains analyzed for the next few hours via functional MRI scanning (fMRI), which measures blood flow in the brain (and therefore brain activity). The Boston University researchers also measured the flow of cerebrospinal fluid (CSF) in the participants’ brains. CSF is a clear water-like liquid thought to purify the brain of metabolic waste material.

The researchers found that sleep actively drives the flow of cerebrospinal fluid through the brain with the help of synchronized waves of neuron (brain cell) activity. The results of this groundbreaking study highlight the importance of sleep in clearing out toxins from the brain — including beta amyloid and tau plaques, which are known to contribute to the progression of neurodegenerative diseases such as Alzheimer’s and Parkinson’s.

Previous sleep science research has shown that the human brain cycles through four stages of sleep. You go through approximately 4 to 5 full sleep cycles each night. You might have heard of rapid eye movement (REM) sleep, the dreaming stage of the sleep cycle. But you also typically go through 3 stages of non-REM sleep before this stage every night. During one non-REM stage of sleep called slow-wave sleep (SWS), neurons (brain cells) undergo slow oscillations in activity, rhythmically switching on and off together. Non-REM sleep has been associated with memory retention and consolidation, and the clearing out of toxins from the brains of mice. However, how this process is linked to brain detoxification in humans has up until this point remained elusive.

With their study, Lewis’ team revealed how neural activity, blood flow dynamics and cerebrospinal fluid are intertwined in the process of brain detoxification during sleep. It turns out that the rhythmic neural activity that happens during slow-wave sleep causes oscillations in blood flow into the brain — when the neurons switch off, they require less oxygen and therefore the flow of blood to that area decreases. In response to this vacuum (absence) of blood, cerebrospinal fluid rushes into the brain.

In other words, purifying waves of cerebrospinal fluid crash through the brain in response to synchronized activation and deactivation of neurons.

“Tonight while you sleep, something amazing will happen within your brain. Your neurons will go quiet. A few seconds later, blood [red] will flow out of your head. Then, a watery liquid called cerebrospinal fluid (CSF) [blue] will flow in, washing through your brain in rhythmic, pulsing waves.” – Kerry Benson, Boston University, Biomedical Engineering News

This research beautifully shows how the phenomenon of slow wave sleep results in the detoxification of the brain at night!

“We know that electrical slow wave activity declines in aging and declines even more so in neurodegenerative disease,” principal investigator Laura Lewis told The Scientist.

The significance of this study is wide-reaching, highlighting the importance of high quality sleep to reduce the impact of aging on cognitive functions such as memory retention. In addition, while this research was limited to the study of healthy young people, it highlights a mechanism that could go awry in neurological disorders, providing a potential target for the development of drugs to treat neurodegenerative diseases such as Alzheimer’s.

To get more brain-detoxifying deep sleep or slow wave sleep, make sure that you give yourself adequate time to sleep each night – at least 7 hours. Getting some intense or vigorous exercise during the day may also increase the time you spend in slow wave sleep.

Citation: N.E. Fultz et al., “Coupled electrophysiological, hemodynamic, and cerebrospinal fluid oscillations in human sleep,” Science, doi:10.1126/science.aax5440, 2019.

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