Listening to brief bursts of pink noise during sleep may help the brain flush out waste products linked to conditions such as Alzheimer's disease, according to a new study. The research, published in Science Translational Medicine, found that precisely timed sound stimulation strengthened slow brainwaves and boosted the flow of cerebrospinal fluid into the brain.
Pink noise, which resembles gentle radio static, has previously been shown to reinforce slow brainwaves during non-REM sleep when played at the right moment. Researchers at Boston University and the Massachusetts Institute of Technology wanted to determine whether this effect extends to the brain's glymphatic system, the network that carries away metabolic waste.
Measuring that connection has been technically difficult because cerebrospinal fluid flow is tracked with MRI scans, which interfere with the EEG recordings used to detect slow brainwaves. The interference made it impossible to align sound bursts with wave peaks in real time using conventional methods.
To solve the problem, the team trained an artificial intelligence model on EEG data collected during previous MRI studies. The model learned to predict when a slow wave peak was about to occur roughly 70 milliseconds in advance, allowing the researchers to schedule sound stimulation accordingly. "This, methodologically, really moves the field forward," said Sephira Ryman of the University of New Mexico, who was not involved in the study.
The researchers recruited 27 healthy adults with an average age of 29 to take afternoon naps inside an MRI scanner while wearing EEG electrodes. Fourteen of the participants managed to fall asleep and reach N2 sleep, a lighter stage of non-REM sleep. "It's a difficult place to fall asleep," said Laura Lewis of MIT, a senior author of the study.
During half of each participant's slow brainwaves, the AI model triggered 50-millisecond bursts of pink noise. The remaining slow brainwaves served as a control with no sound. Analysis of the MRI and EEG recordings showed that the noise bursts strengthened the slow waves they coincided with and briefly increased the flow of cerebrospinal fluid into the brain compared with the silent periods.
The increased fluid flow appeared to be driven by enhanced pumping of blood vessels in the brain, which pushes fluid through the glymphatic system. "We know that the flow in is usually balanced with the flow that comes out," Lewis said, suggesting the boosted inflow also enhanced outflow and waste clearance.
Joshua Levitt of Boston University, the study's lead author, described the sounds as "little staticky beeps" that are barely noticeable. The team is now investigating whether the technique works in older adults and whether it can improve the clearance of proteins such as beta-amyloid, which accumulate in the brains of people with Alzheimer's disease.
If those results are positive, the researchers hope to test whether the approach can slow cognitive decline in normal ageing, mild cognitive impairment, and early-stage Alzheimer's. Previous work has shown that exposing people to sounds and flickering lights while awake may slow cognitive decline in Alzheimer's patients, potentially by boosting the glymphatic system. The new approach could eventually be delivered through a portable device during sleep, which Ryman noted could be less disruptive than awake-state stimulation.