The crash of ocean waves, the babbling of brooks, the pitter-patter of rain on shingles — many people swear by these watery sounds to help them fall asleep and stay in la-la land. Why does flowing “agua” apparently have such a powerful and popular drowsing effect?
Part of the answer lies in how our brains interpret the noises we hear — both while awake and in the dead of night — as either threats or non-threats.
Certain sounds, such as screams and loud alarm clocks, can hardly be ignored. Yet other sounds, like the wind in the trees and waves lapping ashore, we sort of tune out.
“These slow, whooshing noises are the sounds of non-threats, which is why they work to calm people,” said Orfeu Buxton, an associate professor of biobehavioral health at Pennsylvania State University. “It’s like they’re saying: ‘Don’t worry, don’t worry, don’t worry.'”
Louder noises in general, as we’ve all experienced, tend to be harder to sleep through. But perhaps even more important than volume is the character of a sound in how it can trigger the brain’s so-called threat-activated vigilance system and jolt us from slumber.
“The type of noise defines if you will wake up or not, controlling for the volume, because the noise information is processed by our brain differently,” Buxton said.
For instance, although the sounds of crashing waves can vary considerably in volume, with quiet intervals followed by crescendos, the waves’ hubbub smoothly rises and falls in intensity.
That’s in stark contrast to a scream or a ringing phone suddenly piercing a silence, reaching peak loudness almost instantly.
“With a scream or a shout, it’s ‘no noise’ and then it goes directly to high pitch,” Buxton said.
This key acoustic distinction between abrupt threat and gradual non-threat was borne out in a 2012 study by Buxton in a hospital setting. Even at low volumes of around 40 decibels — a whisper, essentially — alarms from hospital equipment aroused study participants from shallow sleep 90 percent of the time, and half the time from deep sleep.
Meanwhile, the sounds of a helicopter and traffic, when reaching the level of a shout at 70 decibels, still did not wake participants as frequently as alarms, ringing phones and even relatively quiet human conversations, which again can feature that jarring, no-noise-to-peak-noise delivery.
We humans, it appears, are biologically hard-wired to respond to noises that come out of nowhere because they can be very bad news.
“We’re mammals, but we’re specifically primates,” Buxton said. “Primates will call to alert their troop about threats,” or, in the case of primitive humans living in groups in the wild, “a scream might be someone in the tribe being eaten.”
In either case, a sudden noise is good reason to stop sawing logs and see what the heck is going on.
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