‘Passive’ screen time may raise dementia risk. How to keep your brain engaged

By Katia Hetter, CNN

(CNN) — It’s not just how much time you spend sitting, but what you’re doing while sitting that may affect your health.

Watching television and other “mentally passive” sedentary behaviors are linked to a higher risk of developing dementia, while more mentally engaging activities such as crossword puzzles appear to offer some protection, according to a new study.

The findings add nuance to what we know about brain health and aging, and they suggest that cognitive activity is important alongside physical activity. To better understand what this research means and how we can better protect our brain health, I spoke with CNN wellness expert Dr. Leana Wen, an emergency physician and clinical associate professor at George Washington University. She previously served as Baltimore’s health commissioner.

CNN: What did this study examine about sedentary behavior, and what were its conclusions?

Dr. Leana Wen: This study, published in the American Journal of Preventive Medicine, followed more than 20,000 adults in Sweden over nearly two decades to examine how different types of sedentary behavior were associated with the risk of developing dementia. Participants were between 35 and 64 years old at the beginning of the study. They reported how much time they spent sitting and what kinds of activities they were doing during that time. This data was correlated with later dementia diagnosis, which was made in 569 individuals during the nearly 20-year study period.

The researchers made an important distinction between mentally passive and mentally active sedentary behaviors. They found that individuals who spent more time in mentally passive activities had a significantly higher risk of developing dementia later. By contrast, time spent in mentally engaging activities while sitting was associated with a lower risk.

They also modeled what might happen if people changed their behavior. Replacing one hour of mentally passive sedentary time with one hour of mentally active sedentary behavior was associated with about a 7% reduction in dementia risk. Adding an hour of mentally active sedentary behavior overall was associated with a 4% reduction, and combining mental engagement with physical activity showed even greater benefit, with 11% lower risk of developing dementia.

CNN: What counts as mentally passive versus mentally active sedentary behavior?

Wen: In this study, mentally passive behaviors were those that involve little cognitive effort or engagement. Examples include watching television or simply listening to music without interacting with it.

Mentally active sedentary behaviors, on the other hand, required attention, problem-solving or interaction. This group included activities like reading, doing puzzles, engaging in hobbies such as knitting or sewing, or working on a computer in a way that involves thinking and processing information.

The distinction is really about whether the brain is being actively used or not. Two people could both be sitting for the same amount of time, but the cognitive impact may be very different depending on the activity.

CNN: Why might mentally passive activities be linked to higher dementia risk?

Wen: One explanation is that the brain benefits from being challenged. Cognitive engagement helps maintain neural connections and may support what’s called cognitive reserve, which is the brain’s ability to adapt and compensate for changes over time.

When people spend long periods in activities that require very little mental effort, those pathways may not be stimulated in the same way. Over the course of months and years, that lack of engagement could contribute to decline in memory and thinking.

Other biological and behavioral factors also may be at play. Mentally passive activities tend to involve long, uninterrupted periods of sitting, which may reduce blood flow to the brain. On the other hand, more engaging activities are often broken up with small shifts in attention or movement, which could be beneficial.

In addition, mentally engaging activities are often more social or interactive, which itself has been associated with better cognitive outcomes. Sleep also may play a role. People who spend more time in passive activities may have poorer sleep quality, and sleep is increasingly recognized as important for memory and long-term brain health.

At the same time, it’s important to note that this study cannot prove cause and effect. It’s possible that people with better baseline cognitive function are more likely to choose mentally engaging activities. So, while the findings are compelling, they should be interpreted as an association rather than proof that choosing passive activities directly leads to dementia.

CNN: What kinds of activities should people consider to keep their brains engaged?

Wen: The key appears to be choosing pastimes that require active thinking. It can be as simple as reading a book, doing a crossword puzzle, playing a strategy game, speaking with a friend or learning a new skill.

Creative hobbies also pose excellent options. Activities such as knitting, drawing, writing or playing a musical instrument all engage different parts of the brain. Even everyday tasks can be made more cognitively engaging by adding an element of challenge, such as trying a new recipe or practicing another language.

You gain additional benefit when these activities involve other people. Social interaction has been consistently linked to better cognitive health and lower dementia risk. Engaging in mentally stimulating activities with others through conversation, group hobbies or shared learning may provide an additional layer of protection.

CNN: Does this mean all screen time is harmful?

Wen: It depends very much on how it is used. For example, watching television for long periods without interaction would fall into the mentally passive category. By contrast, using a computer to solve problems, learn a new language or keep in touch with loved ones can be mentally stimulating.

The concern raised by this study is not so much about screens themselves, but more about prolonged periods of low-engagement behavior. In today’s environment, that could include things such as mindless scrolling or continuously consuming content on social media.

CNN: How important is physical activity compared with mental activity for reducing dementia risk?

Wen: Both are important. Physical activity has well-established benefits for brain health, and a sedentary lifestyle is associated with greater dementia risk. What this study adds is the idea that mental activity during sedentary time also matters. It also showed that combining physical activity with mentally engaging behaviors was associated with an even greater reduction in dementia risk than either alone.

CNN: What is the key takeaway for people who are worried about cognitive decline?

Wen: The key takeaway is that small, everyday choices can make a difference over time. People should aim to reduce prolonged periods of passive behavior and replace them, when possible, with activities that engage the mind.

At the same time, we should remember that brain health is closely tied to overall health. That means staying physically active, maintaining good cardiovascular health, avoiding smoking, following a nutritious diet and getting adequate sleep. Based on decades of research, it’s the combination of these habits practiced over years that helps support cognitive function over the long term.

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