Pink noise-often used to promote sleep-may reduce restorative REM sleep and interfere with sleep recovery. In contrast, earplugs were found to be significantly more effective in protecting sleep against traffic noise, according to new study published in the journal Sleep from the University of Pennsylvania Perelman School of Medicine.
The findings challenge the widespread use of ambient sound machines and apps marketed as sleep aids.
"REM sleep is important for memory consolidation, emotional regulation and brain development, so our findings suggest that playing pink noise and other types of broadband noise during sleep could be harmful-especially for children whose brains are still developing and who spend much more time in REM sleep than adults," said study lead author Mathias Basner, MD, PhD, professor of Sleep and Chronobiology in Psychiatry.
What was tested
Researchers observed 25 healthy adults, ages 21 to 41, in a sleep laboratory during eight-hour sleep opportunities over seven consecutive nights. The participants reported not previously using noise to help them sleep or having any sleep disorders. Participants slept under different conditions, including being exposed to aircraft noise, pink noise, aircraft noise with pink noise and aircraft noise with earplugs. Each morning, they completed tests and surveys to measure sleep quality, alertness, and other health effects.
When we sleep, we cycle multiple times through periods of deep sleep and REM sleep. Deep sleep is important for physical restoration, for memory consolidation, and for clearing toxins from the brain. REM sleep, also called dream sleep, is important for emotional regulation, for motor skills, and for brain development.
This way, deep and REM sleep complement each other, and collectively guarantee that we wake up restored in the morning, ready for the next day.
What is pink noise?
Pink noise is a type of broadband noise-a continuous sound spread across a wide range of frequencies-that sounds uniform and static-like. Broadband noise also includes the well-known white noise and other noise colors like brown and blue noise.
The different noise colors differ in their energy content across the audible spectrum which determines whether they sound high-pitched or low-pitched. Nature sounds like ocean or rainfall sounds are also broadband sounds, and many household appliances like air conditioning units and fans produce broadband sounds.
Eye-opening results
Exposure to aircraft noise-compared to none-was associated with about 23 fewer minutes per night spent in "N3," the deepest sleep stage. Earplugs prevented this drop in deep sleep to a large extent. Pink noise alone at 50 decibels (often compared to the sound of a "moderate rainfall") was associated with a nearly 19-minute decrease in REM sleep.
If pink noise was combined with aircraft noise, both deep sleep and REM sleep were significantly shorter compared to noise-free control nights, and time spent awake was now also 15 minutes longer, which had not been observed in aircraft noise only or pink noise only nights.
Participants also reported that their sleep felt lighter, they woke up more frequently, and their overall sleep quality was worse when exposed to aircraft noise or pink noise, compared to nights without noise-unless they used earplugs.
Habits of millions may lead to bad sleep
The results, the researchers said, suggest not only that earplugs-which are used by as many as 16 percent of Americans to sleep-are likely effective, but also that the overall health effects of pink noise and other types of broadband noise "sleep aids" need to be studied more thoroughly.
Millions of people play back broadband noise during sleep every night. For example, white noise and ambient podcasts accounted for three million daily hours on the Spotify platform, and the top five videos on YouTube to the prompt "white noise" have been watched more than 700 million times. Even so, research on the effects of broadband noise on sleep remains scarce and inconclusive, according to a recent review by Basner and colleagues.
REM sleep disruption is a common feature of disorders such as depression, anxiety, and Parkinson's disease. Basner noted that young children, compared to adults, spend much more time in REM sleep-and thus may be particularly vulnerable to the ill effects of pink noise. Yet, it is common that parents place sound machines next to the bed of their newborns or toddlers, with the good intention to help them fall and stay asleep.
"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," Basner said.
This research was funded by the U.S. Federal Aviation Administration Office of Environment and Energy through ASCENT, the FAA Center of Excellence for Alternative Jet Fuels and the Environment, project 86 through FAA Award Number 13-C-AJFE-UPENN under the supervision of Susumu Shirayama. Any opinions, findings, conclusions or recommendations expressed in this material are those of the investigators and do not necessarily reflect the views of the FAA.
