Watching too much television in your 20s may rewire your brain over time, eventually slowing down processing speeds and overall function, according to a new study published in JAMA Psychiatry. Reportedly the first to examine the long-term effects of being a couch potato, the study reveals harm extends beyond the physical to also affect our mental capabilities.

For the study, a multidisciplinary team of researchers asked 3,247 adults aged 18 to 30 how much TV they watched as well as how physically active they were intermittently over the span of 25 years. Those who reported over three hours of TV viewing in at least two-thirds of the follow-ups were considered “high television viewing.” They also underwent three different tests to measure their cognitive function during the last year of the study —  the first assessed processing speed, the second assessed executive function (decision-making), and the third assessed verbal memory.

Participants who watched high levels of TV during the 25 years were more likely to have poor cognitive speed and decision-making function, but not verbal memory, the tests found. Those who only reported low physical activity levels, the researchers found, scored lowest on only one of the cognitive tests, while those who reported both high television viewing and low physical activity scored twice as high for poor overall cognitive performance as other participants.

"We found that low levels of physical activity and high levels of television viewing during young to mid-adulthood were associated with worse cognitive performance in midlife," the study’s researchers concluded. "Individuals with both low physical activity and high sedentary behavior may represent a critical target group."

Few other studies have investigated how extensive TV viewing can impact a person’s brain on a long-term basis. The researchers believe people with both low physical activity levels and high levels of sedentary behavior may be potential targets for treatment interventions considering they may be at risk for lower cognitive function. According to the Harvard School of Public Health, physical activity doesn’t have to mean going for a run or bike ride; instead it’s any movement that burns calories, whether it’s for work, play, chores, or the daily commute.

Across the world, people are less active today than they were decades ago. Flashback to the United States in the 1950s and 60s, 30 percent of Americans were working in high-activity occupations and 40 percent of American schoolchildren walked or rode bikes to school. By 2001, only 22 percent of Americans had high-activity jobs and only 13 percent of kids walked or biked to school. Physical activity is increasingly being replaced by sedentary activity, from watching television to spending time on the computer.

With low levels of physical activity in the population, there will be more overweight or obese people who are at a greater risk of developing high blood pressure, coronary heart disease, depression, and certain types of cancers, according to Johns Hopkins Medicine. Inactivity also tends to increase with age. The researchers hope that with lower brain speed and function also being a result of greater couch time, public health efforts will go to greater lengths to encourage the public to be active.

Source: Hoang TD and Yaffe K. JAMA Psychiatry. 2015.