Study details

Sounds used for different types of brain stimulation or relaxation are assigned various colors based on how their noise spectrum lines up with a colored light spectrum. White noise, for example, plays all frequencies with the same intensity, similar to how white light combines all visible colors. Pink noise, meanwhile, contains all the frequencies humans can hear but emphasizes lower frequencies and has been compared to the sound of rain or ocean waves.

For the study, researchers at the University of Pennsylvania recruited 25 healthy adults between the ages of 21 and 41 to determine the effects of environmental noise, pink noise, and earplugs on sleep quality. None of the participants in the study had sleep disorders or had previously used ambient sound machines.

Lights out for the study participants was at 11 p.m. each night and participants were awakened at 7 a.m.

During the time when participants were sleeping, they were exposed to either no noise, environmental noise only, pink noise only, a combination of pink and environmental noise at varying decibels, or environmental noise only with earplugs. Environmental noise ranged from traffic sounds to sonic booms.

In a typical night, a person's brain cycles repeatedly through deep sleep and REM sleep. Deep sleep plays an important role in physical recovery, memory processing, and the removal of waste products from the brain, while REM sleep is an important sleep stage for mood regulation and mental focus. Disrupted REM sleep has been commonly seen in conditions like depression, anxiety, and Parkinson's disease.

Discussion

Mathias Basner, professor of psychiatry at the University of Pennsylvania Perelman School of Medicine and lead author on the study, said he didn't expect pink noise to disrupt sleep as much as it did.

"There have been studies that have reported the REM sleep reduction already," he said. "Research has been there, but it has been neglected, and we kind of uncovered that again."

It's unclear why the pink noise affected REM sleep; however, "constant auditory input" could inhibit brain processes that are responsible for sleep, Basner said.

While losing 20 minutes of REM sleep may not seem like a lot, Basner said the lost minutes can add up.

"You may be only losing 10 minutes that night, but then across the week, it's 70 minutes, and across a year, it's 52 times 70 minutes," he said.

Basner added that REM sleep loss could be more pronounced for newborns, as REM sleep makes up half of their sleep compared to just 25% in adults. As a result, Basner cautioned against the use of noise machines for infants and toddlers.

"Overall, our results caution against the use of broadband noise, especially for newborns and toddlers, and indicate that we need more research in vulnerable populations, on long-term use, on the different colors of broadband noise, and on safe broadband noise levels in relation to sleep," he said.

Rafael Pelayo, clinical professor in the division of sleep medicine at Stanford University School of Medicine, noted that since the participants in the study were in a lab, the findings may not apply to the average person at home.

"The need for sleep is biological, but the way we sleep is learned," he said, meaning that people can adjust to any sleep conditions.

If a sound machine helps improve your sleep, Basner said you should play it at a low volume and set a time so it's not playing through the night.

"I don't want to discount that there must be something behind it, because so many people are using it," he said.