This is an excerpt from Fit in 5 by Greg Whyte.
Depression to be second biggest cause of disability worldwide by 2020
Depression is a growing problem throughout the world. Physical activity significantly aids mental health by improving mood and reducing stress and anxiety levels. This gain in physical self-perception and self-esteem is important in reducing the development of mental illness. All forms of exercise can help to prevent or treat mental illness and enhance mental health. And since the effects of exercise are different for each of us, tailoring your program to optimize your enjoyment makes exercise more effective.
There are physiological and psychological reasons why exercise leads to that "feel good state." Physiological factors include the release of chemicals (called endorphins), that lead to a feeling of improved well-being. The release of these naturally occurring opiate-like substances results in what we commonly call "runner's high." Other chemicals, including dopamine and serotonin, which are involved in nerve transport in the brain, are released in greater quantities during exercise and this causes the "buzz" we often experience afterward. It has been suggested that simply increasing body temperature through exercise reduces anxiety, in a similar way as taking a hot bath or a sauna does. In psychological terms, it may be that exercise simply distracts you from the stresses of daily life that create anxiety. Relaxation can have a similar effect, although the impact of exercise lasts longer than relaxation alone. Another psychological factor that improves mental health is the concept of "mastery." When you start a new exercise there is a real sense of achievement and self-satisfaction as you begin to master it. New exercises also make fitness sessions more interesting, and the net result is that you are more likely to maintain your new exercise regimen.
Another important component in mental health is sleep. Unfortunately, roughly one-third of adults in the US report sleep problems. Exercise is closely linked with improved sleep quality. Physically active individuals fall asleep faster, and sleep longer, and more deeply than inactive individuals. So why should we bother exercising? Because, quite simply, it is the easiest, cheapest and most effective way to improve your chances of a long, happy and healthy life.