Information on the physical benefits of exercise abounds. But how does physical activity affect our mental health? Recently, studies have begun looking at the effects exercise has on depression, anxiety and stress, and how exercise helps reduce the decline in our mental state as a result of disease or age. Regular exercise, regardless of the intensity, reduces anxiety and depression levels. Both anaerobic activity (such as sprinting 100 metres) lasting less than five minutes, as well as low to moderate intense exercise (40-60 per cent of maximum heart rate) stimulates anti-anxiety effects in the brain. Although single exercise sessions every few weeks or months can have a positive effect on individuals who suffer from depression, the greatest benefits are to those who consistently exercise for at least 10 to 15 weeks. A study done by California State University found students who worried during exam time suffered less from depression if they exercised. While these studies show physically active people have lower rates of anxiety and depression, researchers are only beginning to understand why. One theory suggests a chemical called norepinephrine may help the brain deal with stress more effectively, and exercise has been shown to increase concentrations of norepinephrine in the region of the brain responsible for stress. Further studies disputed this. Exercise is now thought to reduce anxiety and depression by increasing the body's ability to respond to stress thereby making it more efficient. Also, according to one study, the type of exercise is important. Unforced or self-motivated exercise is more beneficial than forced exercise, due to a combination of biological and social factors. Biologically, exercise allows the body to practise dealing with stress. It forces all the physiological systems involved in stress response to communicate with each other more effectively. So the true value of exercise may be the workings of the body's communication system. If you stop exercising, your body will become de-conditioned and less effective in handling stress. Many elderly people suffer from mental decline including memory lapse or Alzheimer's disease. Two United States studies found people who exercised were less likely to suffer from these types of cognitive decline. In the first study, 106 men and women with an average age of 80 were asked about their participation in physical activity. The women who exercised were better able to focus on tasks, regardless of distractions. The second study looked at how physical activity affected brain functioning in the elderly. Men and women aged between 66 and 89 had their brains scanned to measure their ability to focus and the physically active men fared better. So while exercise has long been associated with physical health, it is now considered just as crucial for mental health. For those of us who spend most of our time sitting at our desks using our brain power, we'd do well to fit in some exercise to help maintain that mental capacity.