Bursts of ‘pink noise’ during sleep seem to help clear waste from brain

Sleeping while listening to bursts of pink noise – background sounds akin to gentle radio static – seems to boost the flow of fluid through the brain, and so might enhance the clearance of waste products that are linked to conditions like Alzheimer’s disease. The pink noise seems to bolster the slow brainwaves that enhance the pumping of blood vessels, which drives the brain’s waste-disposal system.

We already knew that listening to bursts of pink noise during sleep can strengthen slow brainwaves, a kind of electrical activity that occurs during certain phases of non-REM sleep, known as N3 (deep sleep) and N2 (the lighter stage before it). This effect occurs when the bursts coincide with the peaks of slow brainwaves.

“They really don’t sound like much, they’re just little staticky beeps,” says Joshua Levitt at Boston University in Massachusetts.

Prior research has also shown that boosting slow brainwaves – using drugs, for instance – can enhance the flow of the brain’s cerebrospinal fluid (CSF), which bathes the brain and carries away waste products.

But it was unknown whether listening to pink noise has the same effect. That’s because measuring the brain’s CSF flow involves imaging it with MRI scans, but MRI interferes with EEG, the technique used to measure waves of electrical activity in the brain. One consequence of this interference is that slow waves recorded via EEG during MRI scans can’t be detected fast enough to align bursts of pink noise with their peaks.

“You have to do a lot of extensive processing of the EEG signals to detect slow brainwaves, so it’s hard to do that fast enough in real-time,” says Laura Lewis at the Massachusetts Institute of Technology.

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To overcome this problem, Levitt, Lewis and their colleagues used EEG data collected during MRI scans from prior studies to train an AI model to rapidly predict when the brain’s slow waves will peak. “[The AI] says, ‘In about 70 milliseconds, a slow wave peak is coming,’ so now we should schedule our sound stimulation to arrive in accordance with that,” says Levitt.

“This, methodologically, really moves the field forward,” says Sephira Ryman at the University of New Mexico.

The researchers then recruited 27 healthy adults, aged 29 on average, to wear EEG electrodes on their scalp while taking an afternoon nap in an MRI scanner. Of these, 14 managed to fall asleep and enter N2 sleep. “It’s a difficult place to fall asleep,” says Lewis.

At the peak of half of each participant’s slow brainwaves, the researchers used the AI model to play 50-millisecond bursts of pink noise. During the remaining slow brainwaves, they played no noise as a control.

By analysing the MRI and EEG recordings, they found that bursts of pink noise strengthened the slow waves they coincided with and briefly boosted the flow of CSF into the brain, compared with no sonic stimulation. This suggests the pink noise also enhanced the flow of CSF through, and out of, the brain. “We know that the flow in is usually balanced with the flow that comes out,” says Lewis.

The increased CSF flow seemed to be driven by enhanced pumping of the brain’s blood vessels, which is known to push fluid through the brain’s waste-disposal system, called the glymphatic system.

Off the back of these results, the team is exploring whether the approach also works in older adults and whether it can boost the clearance of proteins, such as beta-amyloid, that are linked to conditions like Alzheimer’s disease. If the results are positive, the researchers hope to test whether the technique can slow cognitive decline in people during normal ageing, mild cognitive impairment – a condition that often precedes dementia – and the early stages of Alzheimer’s disease.

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There is reason to think this could work. Exposing people to sounds and flickering lights – while they are awake – has previously shown promise at slowing cognitive decline in people with Alzheimer’s, potentially by boosting the glymphatic system.

But one benefit of the new approach is that it could eventually be delivered via a portable device while people are sleeping, which could be less disruptive, says Ryman.