Study details and key findings

For the study, which was published in Alzheimer's Dementia: The Journal of the Alzheimer's Association, researchers followed over 1,700 adults for almost 24 years, tracking their TV viewing habits in midlife before examining their brains using MRI scans later in life.

The participants joined Atherosclerosis Risk in Communities Study between 1987 and 1989 and were 53 years old on average. As part of the study, they reported how often they watched TV during their leisure time, ranging from "never/seldom" to "very often." Participants also reported how much of their workday they spent sitting.

Overall, the researchers found that participants who frequently watched TV had smaller brain volumes in areas that are vulnerable to Alzheimer's disease. These participants also had larger volumes of white matter hyperintensities, which are areas of damage in the brain's white matter that are linked to aging, stroke risk, cognitive decline, and dementia.

In addition, participants who watched TV very often had smaller frontal and occipital lobes. The frontal lobe plays an important role in executive functions like planning, decision-making, and problem-solving, while the occipital lobe is mainly involved in visual processing.

Notably, these findings were only discovered in male participants in the study. The same associations were not found in female participants. Notably, these associations remained even after researchers accounted for several different health and lifestyle factors, such as physical activity, diabetes, body mass index, smoking, and alcohol use.

Discussion

David Raichlen, professor of biological sciences and anthropology at the USC Dornsife College of Letters, Arts, and Sciences, as well as Natan Feter, a postdoctoral scholar in the Human and Evolutionary Biology program at USC Dornsife and the study's corresponding author, said the findings reinforce the idea that "not all sitting is equal for brain health."

"Most of our waking hours are spent sitting, but our findings suggest that how we spend that time matters," they said.

"Unlike reading, learning a new skill, working, or even using a computer for problem-solving, television often requires little active mental engagement," said Soma Sengupta, chair of neurology and neurologist-in-chief at Tufts Medical Center. "Over many years, less cognitive stimulation could contribute to less 'exercise' for brain networks involved in planning, memory, and attention."

Previous research has found that women tend to prefer watching TV in shorter chunks, though they spend more time watching it overall, while men's preferences for watching sports and movies mean they're more likely to spend longer, uninterrupted periods watching TV.

Hormonal differences, lifestyle factors, or differences in how men and women's brains age could also play a role.

However, Randy D'Amico, neurosurgeon at Northwell's Lenox Hill Hospital, noted that the study is observational and doesn't actually prove that TV causes brain shrinkage. "It's more of a statement that our sedentary time matters," he said.

D'Amico also stressed that TV isn't necessarily bad for your brain overall. "It's just as important to just think about what television is replacing," he said. "If hours of passive screen time come at the expense of exercise, learning, social connection, or sleep, that's where I become concerned. Brain health is about maintaining an active, engaged lifestyle."