New U.S. research has found that just one hour of screen time can affect children's and teens' behavior. Even children as young as two are at risk of higher levels of anxiety and depression due to time spent on smartphones or watching television.

The study, by researchers at San Diego State University and the University of Georgia, looked at data gathered from the National Survey of Children's Health carried out in 2016.

The researchers analyzed 40,337 surveys completed by caregivers of children aged 2 to 17, who were asked about the children's existing medical care, emotional, developmental and behavioral issues, and youth behaviors, including daily screen time.

The findings, published in Preventative Medicine Reports, showed that more hours of screen time are associated with lower well-being in children and adolescents aged 2 to 17, with high users showing less curiosity, self-control, and emotional stability.

Among preschoolers, high users of screens were twice as likely to often lose their temper and 46 percent more likely to be unable to calm down when excited. Around 22.6 percent of those aged 11 to 13 who spent more than seven hours with screens daily were not curious or interested in learning new things, compared to 13.8 percent of those who spent four hours on screen and around 9 percent of those who spent one hour in front of a screen.

Teens who spent more than seven hours a day on screens were twice as likely as those spending just one hour on a screen to have been diagnosed with anxiety or depression, a significant finding according to the researchers. In addition, 42.2 percent of teens aged 14 to 17 who spent more than seven hours a day on screens did not finish tasks, compared with 27.7 percent of those who spent fours hours a day on screens and 16.6 percent for those who spent one hour daily in front of a screen.

Moreover, the associations between screen time and well-being were stronger among adolescents than among young children.

"At first, I was surprised that the associations were larger for adolescents," commented researcher Jean Twenge. "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."

Twenge added that the study provides further evidence to support the American Academy of Pediatrics' established screen time limits -- an hour a day for those aged 2 to 5, with a focus on high-quality programs -- and suggested that similar limits of perhaps two hours a day should be applied to older school-aged children and adolescents.