The average American adult now spends about 6½ hours a day sitting — an increase of about an hour a day since 2007. For teenagers ages 12 to 19, that number is eight hours a day. More time spent on computers gets much of the blame for this trend in sedentary behavior, according to an analysis, published in the Journal of the American Medical Association, of 16 years of data gathered by the National Center for Health Statistics. Although 60 percent of the U.S. population spends at least two hours a day sitting to watch television, according to the report, that percentage has remained stable in recent years. But the numbers for computer use outside of school or work have increased for all age groups. Now, 50 percent of adults spend at least an hour of their personal time each day on a computer, up from 29 percent in 2003, and about 53 percent of adolescents and 56 percent of children now log that much time on computers. In addition, among those who use computers, as many as 1 in 4 do so for three hours or more each day — outside of school or work. The overall time people spend sitting has health experts worried. Research has linked sedentary behavior to an increased risk for obesity, diabetes, high blood pressure, heart disease and more. When people sit and are not physically active, their muscles and bones can weaken, and they are more likely to gain weight because they burn fewer calories. Some experts believe inactivity negatively affects blood circulation and the immune system, as well. Sedentary people also do not get the boost in energy, mood and sleep quality that comes from regular activity.