You Know White Noise, But What's Pink Noise and Brown Noise?

It's the sound inconsistency (noises going from loud to soft or vice versa) rather than the sound level that tends to wake you up. White noise creates a blanket of sounds that masks this inconsistency. White noise is made up of sounds from all over the spectrum, from low-frequency bass notes to high-frequency chimes. These sounds are all blended together to create a constant stream of soft noise. ("Frequency" refers to how fast the waves vibrate per second, according to the Sleep Foundation.)

White noises occurring in nature include sprinkling rain, gently running water and a breeze sifting through trees, all sounds that humans tend to find relaxing at any time of day.

Pink noise is similar to white noise, but it leans less heavily on high frequencies and more on bass and mid-range tones, so it sounds like moderate rainfall or ocean waves. Those who dislike higher-pitched sounds may find pink noise more pleasing to the ear.

Brown noise emphasizes bass notes even further, almost completely eliminating high frequencies from its profile. Natural brown noises can be things like roaring river rapids, heavy rainfall and distant rumbling thunder. This type of noise is named not only for a color, but also for Scottish scientist Robert Brown. In the 1800s, Brown observed pollen particles moving randomly in water and devised a mathematical formula to predict these movements. When this randomizing formula is used to generate electronic sound, a bass-heavy noise profile results. Brown noise is sometimes known as red noise.

There's still a great deal that science doesn't quite understand about human sleep patterns, and the studies on auditory stimulation and sleep have been small. One 2017 experiment at Oxford University on eight sleepers found that subjects fell asleep around 40 percent faster while listening to white noise. Overall sleep time was mostly unchanged, though. A 2016 study showed that 16 young adults had slightly improved recollection of vocabulary words if they slept under pink noise. And another 2017 study at Northwestern University (of 13 older adults) linked pink noise with deeper sleep and improved ability to recall words.

A larger study conducted by the Journal of Caring Sciences in Iran looked at 60 elderly coronary patients, with half of them sleeping under white noise, and half with regular hospital ambient sounds. In the control group, scientists found that quality of sleep degraded as the patients spent multiple nights in the hospital. For those getting the white noise treatment, however, quality of sleep remained roughly the same throughout their stay. There haven't been any research studies on the effects of brown noise on sleep.

The effects of white, pink and brown noises will most likely remain subjective until experiments can be conducted with larger sample sizes and a more diverse array of participants.

"What I tell my patients is, 'I really don't know which is going to be better. Why don't you just try them out to see which is relaxing for you?'" said Dr. Phyllis Zee, director the Center for Circadian and Sleep Medicine at Northwestern University Feinberg School of Medicine, in a CNN article. Zee was one of the researchers in the 2017 study of pink noise and older adults. You could even try blending all three as in the video below.

There are plenty of free and paid options available to try all the various colors for sleep. Natural and synth tracks can be found on streaming services like YouTube and Spotify. There are also dedicated mobile apps for sleep assistance, which can provide a more curated selection of audio. You can also buy purpose-built noise machines, usually with integrated speakers and a multitude of noise profile options.

Many of these have timer functions, to play noise as you fall asleep and then turn themselves off after an hour or so. Whatever the audio source, you'll want to set the volume on the lower end to blend in with regular background ambience. Overly loud noises can make sleep quality worse, so it's best to apply white, pink or brown noise conservatively.