Some evidence for this comes from large-scale, population-based correlation studies. There is strong epidemiological evidence that active persons are less sad than sedentary ones. They also have better psychological health, including fewer anxiety and depressive symptoms. Furthermore, exercise is known to enhance the immune system and reduce the risk of cardiovascular disease, diabetes, some cancers, and dementia. It also plays a role in weight loss and maintenance. Exercise at a moderate intensity for 30 minutes on five days per week is recommended by most health organizations to benefit these conditions and more. Exercise is essential for maintaining optimal mental and physical health.