A rise in the use of computers and social media is having a negative impact on the health of young people, a study suggests.
The report by the World Health Organisation (WHO) says the increased risks of spending too much time on devices such as a tablet or smartphone includes an impact on mental health and sleep deprivation.
The WHO sent questionnaires to more than 200,000 schoolchildren in 42 countries, including England, Scotland and Wales.
The research showed that over 75% of children aged between 11 and 15 years old spend at least two hours using electronic devices on week days.
Among girls aged 15 and over, use of those devices tripled between 2002 and 2014.
It also found that fewer than one in seven girls now meets the daily target of exercising for at least one hour.
A girls school in Kent recently trialled a three day mobile phone and social media ban.
Pupils said it freed up more time for them to pursue other activities such as playing sport.
One pupil at Benenden School said she found she was using her phone "out of habit rather than feeling like I actually need it, so I think it was good to spend some time away".
The headmistress of the school where the experiment was carried out says parents can help children limit the amount of time they spend online.
Samantha Price said: "What I would like to really try to encourage all of us as parents, [is] to think really carefully about the example that we set at home with our devices.
"So for example not having our devices with us while we're eating meals with our children."
Chair of the National Obesity Forum Tam Fry said the sedentary lifestyle that social media can encourage puts young people at risk.
"Girls particularly are not getting the exercise that they should.
"You can't blame them if they find talking to their friends on social media more exciting than exercise, but without the exercise they're not able to burn off the calories and keep themselves in shape which is absolutely fundamental."
Lead author Dr JoInchley, from the University of St Andrews, said the risks associated with spending too much time online have both long and short term implications.
"We also know there are risks, such as cyber bullying and impact on mental health, as well as things like missing out on sleep.
"Also, there are longer-term impacts on physical health from being sedentary."
She said these risks included cardiovascular disease, obesity and Type 2 diabetes.
"It's about reducing time being spent sedentary, and ensuring that children still have opportunity to be active. We really need to start addressing these challenges now," she added.