I had a couple of posts on this a while ago - Get your greens and Exercise in the outdoors - but I spotted this bit of news that says the same thing:
Forests -- and other natural, green settings -- can reduce stress, improve moods, reduce anger and aggressiveness and increase overall happiness. Forest visits may also strengthen our immune system by increasing the activity and number of natural killer cells that destroy cancer cells.
Many studies show that after stressful or concentration-demanding situations, people recover faster and better in natural environments than in urban settings. Blood pressure, heart rate, muscle tension and the level of "stress hormones" all decrease faster in natural settings. Depression, anger and aggressiveness are reduced in green environments and ADHD symptoms in children reduce when they play in green settings.
A nice excuse of one of my photos of the Highlands anyway.