Spending more than 3 hours a day sedentary worsens teens’ mental health

Adolescents who spend more than three hours a day engaged in sedentary behaviors – including playing video games, reading for leisure or spending a lot of time distracted by screens – have a higher risk of facing psychological distress in the future, according to a study published in the Journal of Adolescent Health.

On the other hand, moderate screen exposure (between 60 and 119 minutes per day) invested in educational activities, such as doing homework or attending classes, was considered a “protective” factor associated with less psychological distress.

Sedentary behavior among adolescents has become a growing problem worldwide, with significant implications for the physical and mental health of the population in this age group. Several studies have shown that a lack of physical activity, especially when combined with excessive use of electronic devices, contributes to an increase in problems such as obesity and cardiovascular disease.

In addition, a growing body of research shows that the effects of a sedentary lifestyle are not limited to the physical body, but can also affect mental health, increasing feelings of anxiety and depression, for example.

The study, conducted at the Institute of Psychiatry, Psychology and Neuroscience at King’s College London in the United Kingdom, analyzed information from 3,675 adolescents who were part of the Millennium Cohort Study, a project that follows children born between 2000 and 2002 and maintains a large database.

The analysis included information on sedentary behavior collected at two points in time: when the adolescents were 14 years old and then at the age of 17. In the first phase, the participants completed a diary in which they recorded the different activities they performed every ten minutes. These activities were categorized into broader contexts: general physical activity, time spent sleeping, recreational screen time, non-screen recreational time, and educational sedentary behavior.

At age 17, the same participants reported their psychological distress using a six-question questionnaire about their feelings, using a tool known as the Kessler Scale. The questions included “how often in the past 30 days” did the participant feel nervous, hopeless, restless, depressed, anxious, and worthless. Analysis of the scores, based on the scale, indicated whether or not they were in psychological distress.

According to André de Oliveira Werneck, author of the article and doctoral student at the Center for Epidemiological Research in Nutrition and Health at the School of Public Health of the University of São Paulo (FSP-USP) in Brazil, the fact that the research was based on responses to sedentary behavior recorded in a diary is one of the differences that make the results so relevant.

Werneck explains that there are several ways to measure sedentary behavior. One of them, which is more objective, uses an accelerometer (a type of device that measures how much a person moves), but it cannot distinguish between different sedentary activities, which are very broad.

“Sedentary behavior includes a variety of activities, such as using a computer, watching television, reading, listening to music, or attending class. Most research focuses on analyzing total sitting time, but we can have positive sedentary activities, such as attending class and doing homework, for example. And there are activities that are not beneficial, such as spending too much time on the Internet or playing video games,” he explains.

A second method of measuring sedentary behavior is subjective, in which people answer a questionnaire about how much time they spend sedentary, watching TV, playing video games, working, or studying in a typical week. Nevertheless, it depends on the participant’s memory.

“Having a record of all the activities of these adolescents, formalized in a diary, provides a much more faithful result and has a more reliable accuracy of the different time periods. It’s not common to use this type of tool, precisely because it’s difficult to implement,” says the doctoral student, who carried out the study as part of a research internship funded by FAPESP.

Impact of reading

To analyze the data, the researchers adjusted for several covariates, including gender, parental education, net family income, parental psychological distress, body mass index, physical activity, total sedentary time and depressive symptoms.

After cross-checking the information, they found that the adolescents spent an average of four hours a day in educational sedentary behavior (school, homework) and about three hours a day in screen and non-screen sedentary behavior. Those who spent more than 180 minutes a day on screens for leisure were associated with greater psychological distress at age 17.

Similarly – and surprisingly – the researchers found that those who spent more than three hours a day reading for leisure (especially boys) also reported more psychological distress. According to the study, while previous research has shown that reading is associated with better mental health outcomes and other healthy behaviors, this new research suggests that excessive reading may be harmful in some cases.

One of the hypotheses to explain this finding, says Werneck, is that adolescents who spend many hours reading are “displacing” time that could be spent on activities with face-to-face or outdoor social interactions, which are protective, leading to greater isolation. In addition, it is possible that some of the reading is done on screen devices (cell phones, computers or tablets), which is also harmful – there are studies in adults that link screen reading to poorer sleep because of exposure to blue light.

“This is an unexpected finding in the study, but it’s important to emphasize that very few adolescents spend a lot of time reading for leisure. Our main finding, given the general context, is that more leisure screen time [video games] was associated with worse psychological distress, while more time in educational activities was associated with less distress,” he says.

Professor Brendon Stubbs, who supervised the study, told Agência FAPESP by email that the study revealed several worrying patterns. “We found that adolescents who spent more than three hours a day on screen-based leisure activities showed significantly greater psychological distress three years later. Video games were particularly influential, with each additional hour associated with a 3% increase in psychological distress.”

According to Stubbs, the results suggest a clear dose-response relationship between excessive recreational screen time and future mental health outcomes. “Importantly, this relationship was context-dependent, meaning that educational screen time did not show the same negative effects, highlighting that the problem is not screen use per se, but how and why screens are used.”

How to minimize the impact

Based on the findings, the researchers suggest interventions that could help minimize the negative psychological effects:

• Set clear limits on screen time: Implement guidelines that limit recreational screen time to less than three hours per day, as the study results show that this is when the risks increase significantly;

• Focus on context: Encourage more educational and structured screen activities rather than passive recreational screen time. The study found that educational screen time had no negative effects;

• Balance activities: Promote alternative leisure activities with social interaction components, as isolated screen time can contribute to psychological distress;

• Gender-specific approaches: Consider personalized interventions, as work has found gender differences in effects (e.g., girls were more associated with screen use for Internet browsing, boys for video games);

• Educational support: Since moderate amounts of homework and class time have been associated with less psychological distress, ensure adequate academic engagement;

• Manage and optimize screen time instead of eliminating it altogether.

Werneck emphasizes that sedentary behavior is very complex, and for adolescents, each activity and context needs to be evaluated separately. “We need to focus on interventions that not only reduce sedentary behavior but also reduce it in some specific and very long activities that are more associated with psychological distress,” he concludes.

About FAPESP

The São Paulo Research Foundation (FAPESP) is a public institution with the mission of supporting scientific research in all fields of knowledge by awarding scholarships, fellowships and grants to investigators linked with higher education and research institutions in the state of São Paulo, Brazil. FAPESP is aware that the very best research can only be done by working with the best researchers internationally. Therefore, it has established partnerships with funding agencies, higher education, private companies, and research organizations in other countries known for the quality of their research and has been encouraging scientists funded by its grants to further develop their international collaboration.