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. 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. 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.
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