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Some people prefer to fall asleep to the sounds of silence. But others find it easier to power down, relax or even study when they can drown out their thoughts with background noise — ranging from staticky white noise to the rumblings of a city street; from Beethoven's Ninth Symphony to the quiet rush of a river stream. But is there any reason to choose a particular sound when you're trying to create a calming acoustic environment? Yes, according to a new study extolling the de-stressing virtues of nature. Researchers from the University of Sussex found the sounds of the great outdoors to make for a more relaxing backdrop than both artificial noise and total quiet. They published their findings in the journal Scientific Reports.

Being in nature helps people calm down and cheer up. In studies, natural environments have been linked to health benefits including improved post-op recovery, and reduced pain and anxiety in hospice care. And, in more than one case, study participants have rated nature sounds as more pleasant than man-made ones. But a few different theories have been proposed to explain why nature has the unparalleled effects it does.

In the current study, researchers compared two theories by examining the impact of different sounds on the brain and body. One theory says that nature helps us focus and relax because it gives us a break from paying attention to all the sights, sounds, smells and other stimuli that fill artificial environments. In psych-speak, nature imposes a lighter "attentional load" on us. A second theory, however, says the outdoors are less stressful because we're evolutionarily conditioned to process stimuli found in nature. So, it's not that offices and intersections and bars have so much more going on than natural settings. Rather, it's that humans have adapted to processing a babbling brook but not a symphony of car horns. 

The evolutionary theory held up better in the current study. For it, 17 young-ish adults listened to natural and artificial soundscapes, which were (roughly) five-minute-long compilations of sounds that played for 25 seconds apiece. They also listened to five-minute blocks of silence. 

Researchers performed fMRI (brain scans) and measured physiological activity (heart rate) while participants listened to the soundscapes. While this was going on, participants also had to take tests that gauged their reaction time. FMRI recordings focused on a network of brain regions called the Default Mode Network. Activity in the DMN typically increases during periods of mind-wandering, daydreaming and other times when we're awake but not consciously doing anything. Conversely, DMN activity decreases when we're performing cognitively challenging tasks. Researchers used the reaction-time test as a proxy for attentional capacity.

They also measured physiological activity associated with two different parts of the nervous system: Parasympathetic activity controls functions, like digestion, that occur when the body's at rest. And the sympathetic nervous system gets our hearts racing and sparks other stimulating processes when we go into flight-or-fight mode.   

Here's what they found: In general, nature and artificial sounds had somewhat consistent effects on the brain and body. Brain activity patterns suggested that artificial sounds promoted self-referential, inward thinking (e.g., lying awake at night obsessing over a daytime slip-up) and that nature sounds facilitated relaxed mind-states and enhanced focus on topics other than oneself. Participants also exhibited faster reaction times when they were listening to nature sounds, compared to artificial sounds and silence. This suggested to researchers that artificial sounds co-opted participants' attention in a way that nature sounds didn't. 

Additionally, nature sounds corresponded to greater parasympathetic activity — that's the power-down system — whereas artificial sounds primarily affected heart rate (sympathetic activity). But physiological changes varied on an individual basis, meaning that nature sounds reduced sympathetic activity more for people who started out with higher stress levels. In other words, nature did the most for people who needed it the most. The results supported the second, evolutionary theory because the relaxing effects of nature sounds didn't appear to be solely, or even mostly, a product of attentional strain. Instead, researchers found multiple avenues through which nature sounds helped participants chill out.