Participation in regular physical activity helps build and maintain healthy bones and muscles, control weight, build lean muscle, and reduce fat; reduces feelings of depression and anxiety; and promotes psychological well-being. In the long term, regular physical activity decreases the risk of dying prematurely, dying of heart disease, and developing diabetes, colon cancer, and high blood pressure. Major decreases in vigorous physical activity occur during grades 9-12, particularly for girls; by 11th grade, more than half of female students are not participating regularly in vigorous physical activity. School physical education classes can increase adolescent participation in moderate to vigorous physical activity and help adolescents develop the knowledge, attitudes, and skills they need to engage in lifelong physical activity. Daily participation in physical education class has dropped from 42% in 1991 to 25% in 1995. Television viewing is the principal sedentary leisure time behavior in the U.S., and studies have shown that television viewing in young people is related to obesity and violent or aggressive behavior.