Passing on the gym to snuggle on the couch and binge watch whole seasons of your favorite show this weekend may not bode well for your brain.

In a 25-year study that tracked more than 3,000 young adults into midlife, researchers found that those with the highest television watching and lowest physical activity scored worse on certain cognitive tests than their fit, less TV-addicted counterparts. In particular, couch potatoes had slightly lower brain processing speeds and worse executive function, but they scored just as well as other participants on verbal memory tests. The findings, reported in JAMA Psychiatry, may suggest that such bad TV and exercise habits early in life could set people up for faster cognitive decline in later life, the authors said.

However, the researchers can only speculate on cognitive decline for now, because they only tested cognitive skills at the end of the 25 years—not at the beginning. Therefore, it’s possible that participants with slightly lower cognitive function scores at the end of the study had those same low scores at the start and just enjoy spending lots of time lounging in front of glowing screens. Researchers can't tell from the data as is.

“Hindsight is always better than foresight,” David Jacobs, a study coauthor and epidemiologist at the University of Minnesota, told Ars. The decades-old study was originally designed to look at cardiovascular risks over time, Jacobs explained. Researchers didn’t think of doing full cognitive tests when the study began in the '80s. And at any rate, the cognitive tests given then would likely be out of date and hard to parse now, he said.

Still, the study adds weight to numerous others that have reported links between sedentary lifestyles and relatively low cognitive function later in life. For instance, a study that followed a group of British people born in 1946 found that those most active in their 30s had less cognitive decline when they hit middle age (43 to 53) than their cohorts. Last year, Jacobs and colleagues published another study showing that people who scored high on a cardiorespiratory fitness test given in young adulthood had higher cognitive test scores in midlife relative to counterparts. The new study is one of the first to add television viewing into these correlations.

During the 25-year study, Jacobs and coauthors checked in with participants every few years and asked them about their TV watching habits as well as their exercise level. The 3,247 participants were about 55 percent white, 45 percent black, nearly 57 percent female, and nearly all had at least finished high school. At the start of the study, participants were 18 to 30 years old with a mean age of 25.

At the end of the study, researchers divided participants into either a high TV watching group or a low-to-moderate TV watching group. High watchers reported sitting through more than three hours of TV a day in two-thirds of the check-ins. The researchers then gave all of the participants three types of cognitive tests that assess processing speed, executive function, and verbal memory.

After adjusting for age, sex, education level, smoking, alcohol use, body mass index, and hypertension, Jacobs and colleagues found that both low physical activity and high TV watching linked to lower cognitive scores on the processing speed and executive function tests. Verbal memory scores were not associated with TV or exercise habits, but people who were both physically inactive and TV addicts had the worst scores on the other two tests.

Still, those cognitive differences weren’t mind-blowing; they would not likely affect daily life, Jacobs said. The bigger question is whether those slightly lower scores in midlife set people up for dementia farther down the line, he added.

Though the study only finds a correlation, Jacobs has some hypotheses as to what might cause a link between TV watching, low physical activity, and brain functions. Exercise improves vascular health, which is responsible for delivering nutrients and oxygen to the body including the brain, he said. And watching mindless television may not exercise the same parts of the brain as, say, reading or socializing.

JAMA Psychiatry, 2015. DOI: 10.1001/jamapsychiatry.2015.2468 (About DOIs).