The think-tank examined the relation between social media use (including online time) and mental illness:
While twelve percent of children who spend no time on social networking websites on a normal school day have symptoms of mental ill health, that figure rises to 27 percent for those who are on the sites for three or more hours a day.
There’s no clear indication as to whether the extra time online was a catalyst for mental health issues, or if it was the other way around. The majority of children, whether extreme users or not, reported anxiety whenever there wasn’t an internet connection.
Remember, correlation does not imply causation, but extreme internet use could be used as a check on whether the child may need mental health help.