The brain undertakes a cleaning cycle during sleep, utilizing cerebrospinal fluid (CSF) to remove chemical waste products that accumulate during waking hours. Until recently, the mechanism driving the circulation of this cleansing fluid remained unknown.
A 2025 study conducted on mice, published in the journal Cell, suggests that regular contractions of cerebral blood vessels – stimulated by the periodic release of noradrenaline, a chemical similar to adrenaline – propel CSF throughout the brain. This discovery offers new insights into how the brain clears itself of potentially harmful substances while we sleep, a process vital for overall neurological health.
Pioneering research led by neuroscientist Laura Lewis at the Massachusetts Institute of Technology (MIT) and Massachusetts General Hospital first demonstrated in 2019 how cerebrospinal fluid circulates in waves during sleep, facilitating the removal of toxins. The findings highlighted a previously unknown connection between brain activity and waste clearance.
Researchers observed this phenomenon using advanced real-time magnetic resonance imaging (MRI) techniques, finding that these CSF waves, driven by brain electrical activity and blood flow, are associated with nighttime brain detoxification. This process is believed to be crucial for preventing neurodegenerative diseases.
“We’ve known for some time that there are these electrical waves of activity in neurons,” said Laura Lewis, co-author of the study and an adjunct professor of biomedical engineering at the Boston University College of Engineering and a member of the Center for Systems Neuroscience. “But we hadn’t realized that there are also waves in the cerebrospinal fluid.”
“Your neurons will fall silent. Seconds later, blood will drain from your head. Then, a watery fluid called cerebrospinal fluid (CSF) will enter, traveling through your brain in rhythmic, pulsating waves,” explained a statement from Boston University.
The brain’s nightly repair work is carried out by the glymphatic system, which functions as a cleaning network to eliminate toxic waste. Unlike the rest of the body, where the lymphatic system handles this task, the brain utilizes its own pathway: cerebrospinal fluid (CSF) flowing through tunnels surrounding blood vessels, mixing with interstitial fluid, and collecting proteins and other potentially damaging molecules, such as beta amyloid and tau, associated with the development of Alzheimer’s disease.
This fluid, laden with waste, exits the brain through the same channels, completing an essential cycle for maintaining brain function.
The study published in Science found that during non-REM sleep, CSF moves in large, gradual waves synchronized with brain electrical activity and changes in blood flow. Lewis, also an adjunct professor of biomedical engineering at Boston University, detailed that this wave of fluid appears to push waste and toxins, promoting brain homeostasis.
Until now, scientists had studied brain electrical activity, blood flow, and CSF movement during sleep in isolation. However, the data from MIT suggests these processes are interconnected, collectively contributing to nighttime brain protection.
The research team acknowledged several methodological limitations in their initial work. The sample included only 13 participants between the ages of 23 and 33, which limits the generalization of the findings to older populations or those with sleep disorders.
Direct observation revealed that each CSF wave occurs after a change in neural activity, followed by an alteration in blood flow, and only then by a massive influx of fluid.
According to Lewis, one hypothesis is that when neurons reduce their activity, less oxygen is demanded, and therefore blood partially leaves the involved brain area.
This outflow of blood reduces intracranial pressure, opening the way for CSF to enter, with its flow restoring internal pressure to safe levels.
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