Spending time outdoors is long understood to offer mental health perks, including reduced stress, improved sleep and enhanced cognition.

But these public spaces might also benefit the global economy, new research suggests.

Visits to national parks around the world may result in improved mental health valued at about $US6 trillion, according to a team of ecologists, psychologists and economists from Griffith University in Australia.

"People already visit parks to recover from stress," said lead author Ralf Buckley in a press release. "In healthcare terms, it's patient-funded therapy."

But the value of such preventative therapy had never been quantified on a global scale, he noted. 

Through three pilot studies involving almost 20,000 people -- based on a representative sample of the Australians and interviews at Lamington and Springbrook national parks -- the researchers measured the impact of park visits on quality of life. 

Using well established economic models, they estimated how those benefits would translate into healthcare savings in Australia, and then globally.

They determined that without outdoor parks, the costs of poor mental health could cost Australia alone an additional $145 billion annually.

The research was published Tuesday in Nature Communications.