A research team at the Massachusetts Institute of Technology said a precisely timed 50-millisecond burst of pink noise delivered during deep sleep can increase both brain slow waves and the flushing motion of cerebrospinal fluid, or CSF. The group described the method as the first known way to actively enhance CSF flow during sleep.
The paper was published on Sept. 9 in Science Translational Medicine. It was led by Laura Lewis, an associate professor in MIT's Department of Electrical Engineering and Computer Science, and first-authored by Joshua Levitt. The team said it has already validated the approach in 14 healthy adults and plans to move on to testing in clinical populations.
Lewis said, 「We can increase the amplitude of CSF flushing waves during sleep. To our knowledge, there has not been a way to do this before. Now that we can enhance CSF flow during sleep in healthy adults, we are excited to bring this technology into clinical populations and see what effects it may have.」
Why CSF matters during sleep
According to the study, the brain accumulates waste products during waking hours, including lactate and metabolically worn proteins. During sleep, CSF moves in and out of the brain in repeated waves and helps wash those waste products away, making the process important for brain health.
Lewis's lab has worked on the topic before. In 2019, the team used functional magnetic resonance imaging, or fMRI, to observe CSF oscillations during sleep and found that those oscillations were tightly synchronized with slow waves, the low-frequency electrical activity seen in deep sleep. The new study asked a more direct question: if slow waves are boosted on purpose, can the force of CSF flushing also be increased?
Previous work had already shown that auditory stimulation can enlarge slow-wave amplitude if the sound arrives at exactly the right point in the wave cycle. Lewis compared it to pushing a child on a swing: if the push lands at the right moment, the swing goes higher. The practical problem was how to detect that moment in real time.
How the team timed a 50-millisecond pink noise burst
The sound used in the experiment was a 50-millisecond burst of pink noise. Like white noise, pink noise spans the frequencies audible to humans, but it puts more energy in lower frequencies and sounds softer at higher ones. The result is closer to steady rain or a distant waterfall, which the team said was gentle enough not to wake participants.
To deliver the burst at the peak of a slow wave, researchers had to monitor each participant's electroencephalogram, or EEG, in real time. At the same time, the study also needed fMRI to track CSF movement, and the MRI magnetic field interferes with EEG readings. To deal with that, the team built an algorithm that removed fMRI-related noise in less than 100 milliseconds. It then added a predictive model to estimate when the next slow-wave peak would appear, allowing the pink noise burst to be delivered at the intended instant.
Results from 14 healthy adults
The experiment showed that the brief sound increased both slow-wave amplitude and CSF oscillation amplitude during sleep. The team's fMRI data also suggested a mechanism: slow waves appeared to trigger contractions and expansions in cerebral blood vessels, acting like a pump that pushes CSF through the brain. That finding helps explain why larger slow waves were paired with stronger CSF flushing.
The paper noted that slow waves occur only during non-REM sleep, and they become more prominent as sleep gets deeper.
Early-stage evidence, with open questions around sleep structure
The study remains an early proof of concept. It involved only 14 healthy adults and has not yet tested the method in clinical groups such as people with dementia. Whether the approach can be extended to disease prevention, or even alter disease progression, will require larger studies.
Auditory sleep intervention also comes with unresolved questions. Earlier research found that non-synchronized, continuous pink noise may increase the proportion of deep sleep, or N3, but may also reduce REM sleep by about 19 minutes. That leaves room for debate over how sound-based intervention affects overall sleep structure. The MIT team said the central difference in its work is precise timing at the peak of the slow wave, rather than continuous playback like some sleep apps, which is why so much of the engineering effort went into solving real-time EEG processing delays.
Next steps: clinical testing and a home device
The researchers said they next want to study whether stronger CSF flow leads to more restorative sleep, with a particular focus on insomnia. Another major direction is whether faster waste clearance could slow the course of neurodegenerative diseases such as Alzheimer's disease.
Levitt said, 「Brain waste clearance is very important for Alzheimer's disease and other dementias. Part of the cause of these diseases is the continued buildup of amyloid and tau proteins in the brain. If we can improve the brain's waste-clearance efficiency, we may have a chance to prevent these plaques from accumulating and delay disease onset.」
Levitt has already started work on a company aimed at building a device for home use, such as a headband, so users could enhance CSF flow at home through precisely timed auditory stimulation.
The study was funded by the McKnight Scholar Award, Sloan Fellowship, Pew Biomedical Scholars Award, Simons Foundation, MIT EECS Transformative Research Fund, and the U.S. National Institutes of Health, or NIH.