The study of more than 577,000 children – aged between 11 and 15 – from 42 high-income countries was published in The Lancet.

It found detrimental mental health impacts start after two hours of screen use for girls and after four hours for boys.

Screen use included television, video games and social media, but excluded screen time for academic purposes.

The study’s lead author Asad Khan, from the University of Queensland School of Health and Rehabilitation Sciences, said they measured life satisfaction and psychosomatic health in participants.

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https://www.abc.net.au/news/2021-08-10/qld-screen-times-damaging-teenagers-mental-health-exercise/100365426

Also see:

Dose-dependent and joint associations between screen time, physical activity, and mental wellbeing in adolescents: an international observational study
Summary: Background:
Mental wellbeing in adolescents has declined considerably during past decades, making the identification of modifiable risk factors important. Prolonged screen time and insufficient physical activity appear to operate independently and synergistically to increase the risk of poor mental wellbeing in school-aged children. We aimed to examine the gender-stratified dose-dependent and joint associations of screen time and physical activity with mental wellbeing in adolescents.
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Findings
Our sample included 577 475 adolescents (mean age 13·60 years, SD 1·64), with 296 542 (51·35%) girls and 280 933 (48·64%) boys. The mean reported life satisfaction score (on a scale of 0–10) was 7·70 (95% CI 7·69–7·71) in boys and 7·48 (7·46–7·50) in girls. Psychosomatic complaints were more common among girls (mean 9·26, 95% CI 9·23–9·28) than boys (6·89, 6·87–6·91). Generalised additive model analyses showed slightly non-linear associations of screen time and physical activity with life satisfaction and psychosomatic complaints for girls and boys. Detrimental associations between screen time and mental wellbeing started when screen time exceeded 1 h per day, whereas increases in physical activity levels were beneficially and monotonically associated with wellbeing. Multilevel modelling showed that screen time levels were negatively associated with life satisfaction and positively associated with psychosomatic complaints in a dose-dependent manner. Physical activity levels were positively associated with life satisfaction and negatively associated with psychosomatic complaints in a dose-dependent manner. Joint associations of screen time–physical activity with mental wellbeing showed that, compared with the least active participants with more than 8 h per day of screen time and no physical activity, most of the other screen time–physical activity groups had considerably higher life satisfaction and lower psychosomatic complaints.

Interpretation
Higher levels of screen time and lower levels of physical activity were associated with lower life satisfaction and higher psychosomatic complaints among adolescents from high-income countries. Public health strategies to promote adolescents’ mental wellbeing should aim to decrease screen time and increase physical activity simultaneously.
https://www.thelancet.com/journals/lanchi/article/PIIS2352-4642(21)00200-5/fulltext