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.
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