When you’re asleep, your brain is still keeping tabs on the world outside. Sounds and even smells have been shown to alter brain activity during sleep.
“Evolutionarily, it makes sense,” says Hong-Viet Ngo-Dehning, a lecturer in psychology at the University of Essex in the U.K. “There might be some danger, and we need to respond.” But some sounds might actually nudge us deeper into sleep.
In a recent paper in Science Translational Medicine, researchers report that when they played a type of sound called “pink noise” to people sleeping in an fMRI machine, they could see the fluid surrounding the brain pulsing in response. That might indicate that such noises could provoke changes in the quality of sleep.
The reason sleep feels restorative is still a subject of active research, but scientists are curious whether the movement of cerebrospinal fluid, which might remove biochemical waste products during sleep, could be involved. “A few years ago, we made this discovery using MRI imaging that when people fall asleep, there are these really large waves of fluid flow that pulse around the brain,” says Laura Lewis, a neuroscientist at MIT and an author of the new study. “Ever since then we've been really interested in understanding what controls these waves…and we think it would be really interesting to see if you could maybe make more.”
For this study, Lewis and colleagues drew on prior research using “pink noise,” which is similar to white noise many people use while falling asleep but a bit lower-pitched and more even sounding. “It's less harsh-sounding than static white noise that people might be familiar with,” Lewis says. In 2013, Ngo-Dehning and colleagues found that playing pink noise to people during sleep seemed to improve their performance on memory tasks the next day. In the years since, findings have been mixed, Ngo-Dehning says, but he suspects that this type of noise does do something for the brain, at least under certain conditions.
Lewis was curious to see whether bursts of pink noise, which have been shown in previous work to influence a type of brain waves called slow waves, would change brain fluid flow, too. Her coauthor graduate student Joshua Levitt developed an algorithm allowing him to play 50 millisecond bursts of pink noise precisely as slow waves peaked in the brain’s activity, while people were sleeping in an MRI—a tricky proposition, but one that allowed the team to see what the sound did to fluid flow in the brain. (Levitt and Lewis have applied to patent this technology and hope to commercialize it in a product that could be worn during sleep.)
What the researchers saw confirmed what others have found: Pink noise amplified slow waves during sleep, potentially making sleep deeper. But they also saw that CSF flow increased in the wake of that amplification. “The most exciting part of our results, at least for me, was that we also observed an increase in cerebrospinal fluid inflow from the fourth ventricle into the brain,” says Levitt. “This was the first example that I'm aware of, at least in humans, of some stimulus being able to modulate cerebrospinal fluid flow non-invasively.”
The study is exciting because it shows clearly that pink noise, slow waves, and these pulses of CSF flow are closely linked, says Ngo-Dehning, and it opens the door to studies where researchers intentionally boost CSF flow to see if it can help, say, people with poor sleep or people with neurodegenerative conditions that may be caused by impaired waste removal from the brain. “That would be the next step,” he says.
