Obesity in American shows no signs of slowing, and the reasons why it’s so widespread can be traced to an increasingly sedentary lifestyle that keeps people inactive, and eating, for more hours of the day.
The problem is especially concerning among children and teens, according to the latest study published in Preventive Medicine. The study analyzed data from the National Health and Nutrition Examination survey from 2003-2004 and 2005-2006. More than 12,500 people ages 6 to 84 years wore activity trackers to log how many of their waking hours they spent active and how many they spent sitting.
Not surprisingly, the older people got, the less active they were. The slowdown starts at age 35, a time of increasing demands of work and family responsibilities. By older age, medical issues and chronic diseases may prevent people from being as physically active as they would like. People in their 20s were the only age group to show increases in physical activity—especially in the early morning—compared to adolescents.
The most alarming data came from children and teens. While young children are often thought to be the most active, the numbers showed that rates of exercise actually declined during the teen years. In fact, 19 year olds age spent as much time being inactive and sedentary as 60 year olds. “It was definitely a big surprise,” says Vadim Zipunnikov, the study’s senior author from the department of biostatistics at the Johns Hopkins Bloomberg School of Public Health.
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Children should get at least an hour of moderate-to-vigorous exercise a day, according to the World Health Organization. But the study showed that among kids ages 6-11, 25% of boys and 50% of girls did not meet this recommendation. Even more teens fell short of the guidelines; 50% of adolescent boys and 75% of teen girls weren’t active for at least an hour a day.
Because the people in the study wore activity trackers throughout the day, the data also provide useful insights into when people in different age groups were most likely to be active and sedentary. Children in school, for example, were most active from 2pm to 6pm, which could reflect the fact that they’re sitting in classrooms for most of the day until mid-afternoon. Zipunnikov says that a single window of activity in the late afternoon may not be ideal, and that the data suggest that spreading out activity throughout the day more evenly could help encourage more exercise among kids.
“We could start playing around with times of exercise,” he says. School start times, especially for teens, may also play a role; kids may not be getting as much exercise because they’re not getting enough sleep and are too tired to be physically active. “One of the major contributors to low levels of physical activity in children might be that they don’t get enough sleep,” he says.
The message to exercise more intensely may also be backfiring, Zipunnikov says. Instead, people might be more open to programs that focus on encouraging exercise at certain times of the day: earlier in the day for people with job obligations, later in the morning for older people and throughout the day for children.
This article originally appeared on Time.com