Professor Richard Mitchell of Glasgow University surveyed nearly 2,000 physically active people in the 2008 Scottish Health Survey.
"There was around a 50 percent improvement in people's mental health if they were physically active in the natural environment, compared to those who weren't and exercise in a gym," Mitchell told the Daily Telegraph. "These aren't serious mental health issues, more struggles in general life, things like mild depression, not being able to sleep, high stress levels or just feelings of not being able to cope."
Only activities carried out in a natural environment outdoors were associated with a lower risk of poor mental health, Mitchell said.
Being in areas that have lots of trees and grassy areas help people calm us down and reduce the body's stress response, Mitchell said.