Ambient Noise, Sound Machines, and Sleep Health - The Apeiron Life Perspective

What it is:

The world is a noisy place, but how each of us responds to that noise varies significantly. For highly sensitive people, much in the way that clutter in a workplace might serve as a distraction from the task at hand, certain kinds of ambient noise can activate the nervous system and put us on edge. Arguably, this is most frustrating when we are trying to sleep at night. 

The World Health Organization (WHO) recognizes noise pollution as a contributor to disease. This may occur directly if the noise itself is a source of stress, but there may also be an indirect path by way of sleep disturbance. In an increasingly busier, louder, 24/7 society, the key noise contributors are air, road, and railway traffic.

The purported claims:

• Ambient noise can activate our sympathetic nervous system, resulting in the release of stress hormones.

• Sound machines may mask disruptive environmental sounds and improve sleep quality, but there is no “one-size-fits-all.”

What the science says:

How We Perceive Sound

Complex biological machinery allows us to identify different sounds in our environment. First, a sound wave is produced by, say, a barking dog. This wave travels towards our ears, passing along the ear canal until it reaches the ear drum. This thin membrane vibrates against three tiny bones in the middle ear, which amplify the incoming sound, causing fluid in a nearby structure called the cochlea to vibrate, in turn activating little hair cells. As these hairs move, they open little pores that allow chemicals to rush into the cells. This initiates an electrical signal which is interpreted by the auditory nerve, which transports the information to the brain stem, and then onwards to the temporal lobe of the brain where it is interpreted as “barking dog” as opposed to, “car” or “bird”, for example. 

The auditory system is, of course, very important. It both enriches our lives (such as when we enjoy music or laughter) and alerts us to potential threats. But if we are bombarded with too much of the wrong type of sound, it can be a source of stress. A stressful noise can be one that is too loud, at an unpleasant frequency (like a high-pitched scream), or jarring, such as a jackhammer or sirens that we can’t tune out. When we perceive a sound as stressful, it activates our sympathetic nervous system. This can trigger the release of stress hormones, including cortisol, and some of the side effects are an increase in oxidative stress, vascular inflammation, and heart rate. If sustained, we might feel irritable, stressed, or anxious without necessarily being consciously aware of why we feel that way.  

Research on mice has shown that four days of exposure to aircraft noise (85 dB) resulted in elevated systolic blood pressure, norepinephrine, and endothelial cell dysfunction. It has been shown that humans exposed to high levels of ambient noise have greater activity in the amygdala, a brain region involved in emotion, fear, and learning.

The sleeping body still responds to environmental stimuli, such as noise, even if we don’t consciously wake up and register it. Experiments have shown that even when the brain doesn’t awaken, the body can still increase heart rate and blood pressure in response to sound. The deeper we fall into sleep, the higher our arousal threshold becomes. This means that it takes more of a stimulus (such as sound or touch) to wake us up. Given how important sleep is for our health, it makes sense that our brain would develop a means of protecting itself from environmental disturbances. We know from decades of data that sleep disturbances are associated with an increased risk of cardiometabolic disorders, such as obesity, diabetes, and heart disease. However, in an emergency, we need to still be able to wake quickly. It has been shown that even in the depths of sleep, hearing a whisper of your own name is more likely to wake you up than a much louder but neutral source of sound.

Masking environment noise with ambient sound

Sound machines are becoming increasingly popular, and for good reason. Intermittent noise from others in the home or neighbors, for example, can stir us from our slumber. Blanketing the environment with a continuous sound, such as the whirring of a fan or hum of an air conditioner, can mask startling noises and allow us to sleep better. 

Several studies have tested the effects of continuous noise on various dimensions of sleep health. The interventions typically use white or pink noise, but there are many other “colors” of noise available on today’s sound machines and apps. These are described below: 

• White – White noise is similar to white light in that it distributes signal power equally across the spectrum. This creates an audible, consistent, flat hissing sound. This sound, like the hum of an air conditioning unit, can be helpful for people with tinnitus.

• Pink – With more power at lower frequencies and less at higher frequencies, this sounds deeper and more balanced than white noise (example: steady rainfall)

• Brown – This is weighted towards the lower end of the spectrum, with a low rumbling or roaring effect (example: thunder)

• Green – Weighted towards the middle part of the audible spectrum, this is considered the most representative of sound in nature, by some (example: a gentle stream)

• Blue & Violet – Dominated by high-frequency sound, this is a hissing noise (example: hissing garden hose)

• Gray – This is like the opposite of green noise, balanced by low- and high-end noise with less from the middle part of the spectrum (example: general environmental sound)

Many studies have measured whether, compared to a control condition with no source of continuous noise, sound did, in fact, make participants fall asleep quicker and/or stay asleep. This has been tested in people of all ages, and there are many mixed findings, with plenty of studies showing no significant differences between conditions. 

It’s important to consider that many of these studies have brought people into a sleep laboratory where they are hooked up to a computer with many electrodes for monitoring. People generally don’t generally achieve their best nights of sleep under those conditions, so this may contribute to many “null” findings. The alternative way of gathering sleep data is via at-home wearable sleep trackers, such as rings and watches. While these devices are improving each year, their ability to accurately measure the duration spent in different sleep stages is imperfect. Therefore, the best way of gauging the effectiveness of different types of sound is probably by simply asking people how they feel. Do they wake up feeling refreshed and well-rested? Do they recall how many times they woke up the night prior? These kind of self-reported measures of sleep are often just us – if not more – important than what a tracker measures.

The studies that have attempted to look at the effects of continuous noise on sleep have used noise varying from 20dB (the volume of a whisper) to 93 dB (the volume of a motorcycle engine). This means that it’s challenging to compare findings. The potential benefits seen on a busy hospital ward may not translate to a quiet home setting. 

One study found that broadband noise (i.e. sound that has a continuous distribution of sound at frequencies exceeding 100 Hz) was just as effective as lullabies at decreasing heart rate, the rate of respiration, and improving sleep quality among premature infants. Ambient sound like broadband noise may serve to mask other sounds that are potentially disruptive in the environment. But we may also use it as a conditioning tool, such that once we hit “play” at night, it becomes a sound that we learn to associate with sleep, cuing us to drift off. 

Some studies have evaluated the effects of acoustic stimulation on memory, finding that there seemed to be benefits for declarative (i.e. remembering names and facts), but not non-declarative memory (i.e. motor skills, habits) in response to specific sound cues administered during sleep. These studies typically employ pulses of pink noise with the goal of enhancing slow wave activity during natural periods of deep sleep. The number of slow waves generally doesn’t increase in response to the sound, but stimulation can sometimes increase the amplitude of existing waves, which may be beneficial for memory consolidation.

Our take:

Some people prefer sleeping without any masking noise so that they can be alerted to potentially important sounds in the night. Others have no choice, such as new parents, who must be ready to care for a baby at a moment’s notice. For others, earplugs offer an optimal solution for tuning out environmental noise. 

When deciding whether to use a sound machine, consider whether ambient noise is truly the thing that’s keeping you up at night. Continuous sound is probably most beneficial in already noisy environments, such as busy hospital wards, urban environments with a lot of traffic noise, or when traveling and sleeping in an unusual place with unusual sounds. However, if we’re fortunate enough to have a quiet place to rest our heads most of the time, “colored” noise may be more disturbing than anything else. Instead of using colored noise, something more natural, such as chirping crickets or rainfall, may be more comforting and in keeping with the environment.

Will this benefit me?

The best way to determine whether a sound machine improves your sleep is simply to try one and see how you feel, experimenting with different types of sound until you find one or two that you like. Consider that it might take a bit of getting used to and think about how it might also be beneficial for other times you would like to rest during the day, such as during meditation or non-sleep deep rest. Most people find that static-like sounds (such as white noise) aren’t particularly beneficial for waking rest, so instead, consider gentle instrumental sounds or environmental soundscapes to transport your mind to a place of deep rest when you’re trying to stay awake. 

If you want to experiment on yourself and you already wear a sleep tracker, consider not so much the sleep data but whether the exposure to continuous sound at night improves your resting heart rate or heart rate variability – a sign that it could be soothing your nervous system and helping you achieve better rest.

Still curious to try it? If you do, here’s what to keep an eye on:

If you decide to try a sound machine at night, be sure that it isn’t excessively loud, and take care to give your ears a rest from continuous music during the day. You can also pay attention to how various sounds may infiltrate your dreams. Experiments have shown that word cues delivered during rapid eye movement (REM) sleep (when creative dreams are most likely to occur) can influence the content of those dreams. If you keep a dream journal, see whether any sounds from a sound machine influence your dream themes. It might even be the case that disruptive sounds from the environment (such as sirens, traffic, or barking dogs) were contributing to negatively toned dreams, and these improve once continuous noise masks the “problem” sounds.

If you’re curious about where to start, a Wirecutter article reviewing different noise machines can be found in the References section below.

References and additional materials:

How we hear and the neural coding of sound - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5819010/

Basics of sound, the ear, and hearing - https://www.ncbi.nlm.nih.gov/books/NBK207834/

Sensory gating during sleep - https://pubmed.ncbi.nlm.nih.gov/38286435/

Environmental noise and sleep - https://pubmed.ncbi.nlm.nih.gov/35857401/

Discriminative responses to noise during sleep - https://academic.oup.com/brain/article/83/3/440/391563

Environmental noise and sleep 2 - https://pubmed.ncbi.nlm.nih.gov/17317241/

Noise and cardiovascular health - https://pubmed.ncbi.nlm.nih.gov/37422031/

Acoustic stimulation and memory - https://pubmed.ncbi.nlm.nih.gov/34850995/

Slow wave stimulation with sound - https://pubmed.ncbi.nlm.nih.gov/23583623/

Noise as a sleep aid review - https://pubmed.ncbi.nlm.nih.gov/33007706/

Infant sleep and broadband sound - https://pubmed.ncbi.nlm.nih.gov/23589814/

Wirecutter noise machine review - https://www.nytimes.com/wirecutter/reviews/best-white-noise-machine/

Image created using BioRender.com