Short sounds can help the brain cleanse itself during sleep

Pink noise is similar to white noise, but in it, low frequencies sound louder, while high frequencies sound quieter. The result is a calm, balanced sound reminiscent of the steady noise of rain or a distant waterfall. In the experiment, very short pulses were used — lasting only 50 milliseconds. They were quiet enough not to wake a sleeping person.

The main trick was to deliver the sound at exactly the right moment — at the peak of the slow brain waves characteristic of deep sleep. These waves are closely linked to the movement of cerebrospinal fluid: when slow waves occur, the brain’s blood vessels constrict and dilate, acting like a pump and pushing out the cerebrospinal fluid. The stronger the slow wave, the larger the cerebrospinal fluid wave becomes, which means that brain cleansing is more active.

To capture the right moment, scientists simultaneously monitored the brain’s electrical activity using EEG and the movement of cerebrospinal fluid using fMRI. This turned out to be a challenging task: the magnetic fields used in fMRI interfere with EEG signals. Researchers developed a method to clean EEG data of these interferences very quickly, in less than 100 milliseconds. In addition, they created an algorithm that predicted when the slow‑wave peak would occur, so that they could deliver an auditory pulse in time. This approach is called a closed‑loop system: the system listens to the brain in real time and responds at the right moment.

Fourteen healthy volunteers participated in the trials. It turned out that properly synchronized bursts of pink noise actually enhance slow brain waves and increase the amplitude of cerebrospinal fluid waves. Thus, for the first time, scientists have been able to purposefully enhance the brain’s natural cleansing mechanism during sleep.

Now, researchers want to find out whether such enhanced cleansing can improve cognitive functions and memory, as well as slow down the progression of neurodegenerative diseases. The fact is that diseases such as Alzheimer’s are partly associated with the accumulation of harmful proteins in the brain, for example, beta‑amyloid and tau protein. If it is possible to improve the elimination of these substances, it may be possible to сдерживать the formation of pathological plaques.

One of the authors of the study, Joshua Levitt, has already founded a company that plans to create a device for home use, for example, in the form of a headband. It will track sleep phases and deliver sound stimuli at the right moment, helping the brain to cleanse more effectively.

According to the study’s senior author, Laura Lewis, the ability to enhance cerebrospinal fluid flow during sleep in healthy adults opens the way to testing this technology in patients with various disorders. First and foremost, scientists want to study its potential for people with insomnia, as well as for those with diseases that impair the brain’s ability to eliminate toxic metabolic waste products.