Why does the sound of running water help us sleep better?

The sound of ocean waves, the murmur of a stream, the patter of rain - many people say that these sounds of water help them fall asleep easily. Why is that?

Part of the answer lies in how our brains interpret the noises we hear [both while awake and late at night] as threatening or not.

" These sounds are non-threatening, which is why they have a calming effect on people , " said Orfeu Buxton, associate professor of health at Pennsylvania State University. " They're like saying, 'Don't worry! Don't worry! '"

Louder noises tend to make it harder to fall asleep. But perhaps more important than volume is the quality of the sound (in other words, whether it triggers the brain's threat alert system and wakes us from sleep).

Non-threatening sounds help calm people.

" The type of noise will be the deciding factor in whether you wake up or not , because our brains process noise information differently ," said Mr. Buxton.

For example, although the sound of crashing waves may vary in volume, it is in stark contrast to a scream or a ringing phone that suddenly cuts through the silence and reaches its highest volume almost instantly.

" In the case of screaming, it's not what you would call 'normal noise' anymore, but it goes straight to high-pitched sound ," said Mr. Buxton.

According to a study by Professor Buxton at the hospital, even at a low volume of around 40 decibels – essentially a whisper – the alarm from the hospital device woke study participants from sleep (whether it was light or deep sleep).

We humans, biologically, seem to be programmed to react to sudden noises because they can signal very bad news.

" We are mammals, but primates ," says Buxton, " and primates will warn their fellow humans of threats ." In the case of early humans who lived in small groups, " a scream might signal that someone in the tribe was being eaten ."

Another reason why the sound of water helps us sleep is that non-threatening noises, when played at a relatively high volume, can drown out sounds that activate the brain's threat alert system, says Buxton .

" It can help to suppress other sounds that you can't control, such as when someone is flushing the toilet in another part of the house, or when there are taxis or traffic outside. " - Mr. Buxton said.

All of which makes it understandable that water-based sleep aids have been popular for decades, in media ranging from cassette tapes to MP3s and, today, mobile apps.

However, based on his own research and other studies, Professor Buxton also warns against over-reliance on mobile devices.

' Your phone is hardly a protector of your privacy and peace ,' says Buxton. ' You may think you've turned off all your notifications, all your text messages and updates, but if your phone isn't completely off, you're still vulnerable to unwanted interruptions .'

Some people also report feeling the need to go to the toilet when they hear the sound of running water. Professor Buxton says that unless this is the case, you should continue to enjoy the gentle melody of the water to drift off to sleep.