Spending more time in green spaces might be just what the doctor ordered, according to a study that quantifies the health benefits of natural sounds, from improved mood and cognitive performance to decreased heart rate and sensitivity to pain.

The paper was led by Rachel Buxton, a conservation biologist at Canada's Carleton University, and published Monday in the Proceedings of the National Academy of Sciences (PNAS).

"We tend to look at the acoustic environment from the perspective of noise pollution and how it disturbs wildlife, and how it has effects on visitors to parks and human health," she told AFP.

"A lot of us conservation biologists are really interested in the inverse of that: what are the health benefits of natural acoustic environments?"

Buxton and her colleagues carried out a literature review to statistically analyze the patchwork of prior research in the area, which was mostly carried out in lab or hospital settings with sounds played to volunteers by headphones.

Seven of the 36 studies examined traditional health outcomes, including heart rate, blood pressure, perceived pain, skin conductance -- which is linked to emotional arousal -- levels of the stress hormone cortisol,  and t-wave amplitude, a measure of heart health.

The rest measured potential precursors along the pathway to health, including metrics of perception, mood, and cognitive performance.

Overall, there was an 184 percent improvement in health and other positive affect outcomes in the groups exposed to natural sounds relative to comparison groups.

The natural sounds also led to a 28 percent decrease in stress and annoyance.

Among the natural sounds played to the volunteers, the sound of water had the highest impact, followed by birdsong, then a mix of both.

The team also examined audio recordings from 68 US national parks recorded over the past 15 years at 221 different locations.

What drives the effect? No one knows for sure, but Buxton has a hypothesis: "Evolutionarily, an acoustic environment that has lots of natural sounds is a good indicator of a safe environment -- so it allows for mental recuperation because we're not on edge."

The findings come at a time when experts say the Covid-19 pandemic is driving a mental health crisis, with the number of people reporting symptoms of anxiety and depression on the rise.