City dwellers have a higher risk for anxiety, depression and other mental illnesses than people living outside urban centers. That is an observational fact but causes behind that are not known.
Gregory N. Bratman, of Emmett Interdisciplinary Program in Environment and Resources, Stanford University, conducted a controlled experiment to investigate whether a nature experience would influence repetitive thoughts focused on negative aspects of the self, a known risk factor for mental illness.
His results indicate that accessible natural areas may be vital for mental health in our rapidly urbanizing world. Even short breaks of several minutes from an office setting can have a very positive effect on attitude - plus the benefits of periodic physical activity.
Here is his article: http://www.pnas.org/content/112/28/8567.abstract