Trying to protect children from all online risks may be futile, according to a new report, which says youngsters are increasingly conducting their online lives in private.
The study also warns that more than a third (37%) of UK 15-year-olds are now "extreme internet users", who are on their computers for more than six hours a day outside of school hours.
This is some of the heaviest use for countries in the Organisation for Economic Co-operation and Development (OECD), with only children in Chile using the internet more.
The report, from the Education Policy Institute (EPI), said young people in the UK are also extensive users of social media sites, such as Facebook, Instagram and Snapchat, with 95% of 15-year-olds in the UK using social media before or after school in 2015.
It added: "This online activity is increasingly private, with young people using the internet in their bedrooms or on a smartphone.
"The rise in popularity of instant messaging means that online discussions are now often held in private groups, rather than on public profiles."
Experts behind the study said that the focus of public policy must be on making children resilient so they know how to navigate the risks, which may include a negative impact on their mental health and exposure to cyber-bullying.
As well as conducting online lives in private, "the way in which young people connect with social media is changing rapidly due to the fast pace of technological innovation, such as the development of livestreaming," it said.
"It is therefore likely to be futile to attempt to protect children and young people from all online risks.
"This indicates that the focus of public policy should be on how to develop resilience in young people to maintain their emotional and mental well-being and to live safe digital lives."
The report pointed to one study from the Office for National Statistics in 2015 which found that there was a "clear association" between time spent on social media and mental health problems.
"While 12% of children who spend no time on social networking websites have symptoms of mental ill health, the figure rises to 27% for those who are on the sites for three or more hours a day."
The report also found that children classed as "extreme internet users" were more than twice as likely to report bullying than moderate internet users.
Overall, a third of UK children have experienced cyber-bullying or been exposed to excessive internet use, or the sharing of private information and harmful content - such as websites that promote self-harm, it said.
The report went on: "Young people use a range of coping mechanisms to deal with online risk, such as blocking other internet users, changing privacy settings or taking a break from the internet.
"Many children do not choose to talk to their parents or a teacher. In one UK study, only one in five children (22%) who were upset by something they had seen online talked with someone else face to face (about it)."
But experts behind the study said there is evidence of a beneficial impact of social media on young people's emotional wellbeing and pointed to a lack of evidence of a direct link between internet use and poor mental health.
They said the internet enables young people to connect with others to improve their social skills online, develop their character and resilience, and collaborate on school projects.
Those with mental health problems can also seek support on the internet, either through social media networks or through the online groups.
Emily Frith, director of mental health at the EPI, said: "This report highlights how social media, when used in moderation, can have a beneficial impact on young people.
"While we also find a negative link between excessive social media use and young people's mental well-being, there is no evidence that it is the direct cause of such problems.
"Our research highlights the importance of equipping young people with skills that help them counter emerging online risks.
"That doesn't mean protecting them from the internet but rather putting forward proactive measures centred on resilience-building - an approach that is vital in helping young people lead safe digital lives".
Dr Bernadka Dubicka, from the Royal College of Psychiatrists, said: "Whilst these technologies can be very beneficial, the high number of young people who are extreme users in the UK is of concern, in light of the association with mental health problems.
"Young people benefit from guidance on the public nature of online interactions, and can benefit from supervision in the same way that they would in real world interactions with peers.
"We would support the call for more work on resilience and education, as well as the need for research to better understand the impact of social media on young people's lives."
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