Americans are becoming increasingly sedentary, spending almost a third of their waking hours sitting down, and computer use is partly to blame, a new study found.

Over almost a decade, average daily sitting time increased by roughly an hour, to about eight hours for U.S. teens and almost 6-1/2 hours for adults, according to the researchers. That includes school and work hours, but leisure-time computer use among all ages increased too.

By 2016, at least half of American kids and adults spent an hour or more of leisure time daily using computers.

The biggest increase was among the oldest adults: 15% of retirement-age adults reported using computers that often in 2003-04, soaring to more than half in 2015-16.

Most Americans of all ages watched TV or videos for at least two hours daily and that was mostly unchanged throughout the study, ranging from about 60% of kids aged 5 to 11 up to 84% of seniors.

“Everything we found is concerning,” said lead author Yin Cao, a researcher at Washington University’s medical school in St. Louis. “The overall message is prolonged sitting is highly prevalent,” despite prominent health warnings about the dangers of being too sedentary.

The researchers analyzed U.S. government health surveys from almost 52,000 Americans, starting at age 5, from 2001-2016. Total sitting time was assessed for teens and adults starting in 2007. The results were published last month in the Journal of the American Medical Association.

Studies have shown that prolonged periods of sitting can increase risks for obesity, diabetes, heart disease and some cancers. U.S. activity guidelines released last fall say adults need at least 150 minutes to 300 minutes of moderate to vigorous activity each week, things like brisk walking, jogging, biking or tennis.

Muscle strengthening two days weekly is also advised. Immediate benefits include reduced blood pressure and anxiety and better sleep. Long-term benefits include improved brain health and lower risks for falls.

Kids age 6 through 17 need 60 minutes of moderate to vigorous physical activity daily. Regular activity is even recommended for kids as young as 3. But only about 1 in 4 U.S. adults and 1 in 5 teens get recommended amounts.

College student Daisy Lawing spends a lot of time sitting, but says she doesn’t have much choice. Classes and homework on the computer take up much of her day.

“I always feel bad” about being inactive, she said recently at an Asheville, N.C., cafe, explaining that she did a school paper about the benefits of physical activity.

“I try to walk a lot, try to work out twice a week. But sometimes I can’t because I’m too busy with school,” said Lawing, 21, a junior at Appalachian State University in Boone.

Peter T. Katzmarzyk of the Pennington Biomedical Research Center in Baton Rouge, La., said people who sit all day need to do more than the minimum recommended amount of physical activity to counteract the harms of being sedentary.

“We’ve just got to really work on the population to get the message out there. Physical activity is good for everyone,” he said.