For kids with ADHD, regular 'green time' is linked to milder symptoms: (University of Illinois at Urbana-Champaign) A study of more than 400 children diagnosed with Attention Deficit Hyperactivity Disorder has found a link between the children's routine play settings and the severity of their symptoms, researchers report. Those who regularly play in outdoor settings with lots of green (grass and trees, for example) have milder ADHD symptoms than those who play indoors or in built outdoor environments, the researchers found. The association holds even when the researchers controlled for income and other variables.
I always suspected that ADHD is just a symptom of activity deprivation. Although this might be a bit too simple, it seems to be partly true.