Researchers from San Diego State University and the University of Georgia have studied the effect that screen time has on children's and teenager’s mental development. The study found that too much TV, gaming and smartphone exposure is linked to high levels of anxiety and depression. In people young than 18, just one hour of screen time was found to cause impaired curiosity, self-control and emotional stability and an increased inability to finish tasks.

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"Previous research on associations between screen time and psychological well-being among children and adolescents has been conflicting, leading some researchers to question the limits on screen time suggested by physician organizations," wrote researchers Jean Twenge and Keith Campbell.

The National Institute of Health estimates that youths spend five to seven hours of their downtime on some type of screen. This year the World Health Organization included gaming disorder in their 11th edition of the International Classification of Diseases. There is a growing link between the declined mental health of children and screen time.

The team used data from the 2016 National Survey of Children’s Health and analyzed over 40,300 surveys from caregivers of children aged two to 17. They also used data from a U.S. Census Bureau nationwide survey on children and screen use. The survey also covered existing medical care, emotional, development and behavioral issues, youth behaviors and daily screen time. The survey excluded youths who have autism, cerebral palsy and other developmental delays that impact a child’s daily functioning.

The study found that teens who spend seven or more hours a day on screens are twice as likely to be diagnosed with anxiety or depression than teens who only spend an hour per day looking at a screen.

"At first, I was surprised that the associations were larger for adolescents," Twenge said. "However, teens spend more time on their phones and on social media, and we know from other research that these activities are more strongly linked to low well-being than watching television and videos, which is most of younger children's screen time."

Four hours of screen time a day was associated with lower psychological well-being in children, compared to one hour of screen time which has the least negative effects on well-being. Preschoolers who use screens frequently throughout the day are twice as likely to lose their temper and 46% more likely to be unable to calm down when they are excited. Teens aged 14 to 17 who spend 7 hours or more a day on screens are 42.2% more likely to have trouble completing tasks than teens who spend less than 7 hours. More than 22% of teens who spend 7 or more hours on screens were uninterested in learning new things.

The team says that the way to fix these problems is to establish clear rules and limits to the amount of screen time a child is allowed a day - including young children and teens.

“Half of mental health problems develop by adolescence," said the researchers. "Thus, there is an acute need to identify factors linked to mental health issues that are amenable to intervention in this population, as most antecedents are difficult or impossible to influence," they continued. "Compared to these more intractable antecedents of mental health, how children and adolescents spend their leisure time is more amenable to change."

The study was published in the Preventative Medicine Reports.