Too much time spent online could be causing mental illnesses and social problems for children, it has been reported.
Loneliness, depression, anxiety, low self-esteem and heightened aggression are some of the possible issues faced by children who may overuse the internet.
Public Health England (PHE) says there is an association between the amount of time spent on social media sites and "lower levels of well-being", the Daily Mail reported.
The newspaper said the link becomes striking when children spend more than four hours a day in front of a screen - but it kicks in even at very low levels of use.
A PHE report says improvements in children's wellbeing over the past two decades had now been "curtailed and may be in reverse".
One in 10 children now has a mental health issue, and a third of teenagers feel "low, sad or down" at least once a week, the Mail said.
The report says the number of calls to ChildLine about online bullying, suicidal thoughts and self-harm have soared in recent years.
PHE suggested that parents should be particularly worried about their child's mental health if they spend more than four hours a day in front of a screen.
"Children who spend more time on computers, watching TV and playing video games tend to experience higher levels of emotional distress, anxiety and depression.
"This relationship is particularly negative among those who engage in high levels of screen use - more than four hours a day.
"The evidence suggests a 'dose-response' relationship, where each additional hour of viewing increases children's likelihood of experiencing socio-economic problems, and the risk of lower self-esteem," the report said.
It added: "Increased screen time and exposure to media is associated with reduced feelings of social acceptance, and increased feelings of loneliness, contact problems and aggression.
"Certain internet activity (social network sites, multi-player online games) have been associated with lower levels of wellbeing."
The report suggests that children's mental health has been getting worse in recent years.
"The data shows overall most children and young people are satisfied with their lives, but that there may be a decline," it said.
"One-fifth report poor well-being and there are important differences for different groups and ages of children and young people. Analysis of the British Household Panel and Understanding Society survey shows that the rise in children and young people's well-being from 1994 to 2008 has curtailed and may be in reverse.
"Peak onset of mental ill health is eight to 15 years. Ten per cent of children have a mental health issue and half of lifetime mental ill health starts at age 14.
"The Health Behaviour of School-Aged Children Survey found that around 30% of English adolescents reported a level of emotional wellbeing considered as (sub-clinical) 'low grade' poor mental health - that is, they regularly (at least once a week) feel low, sad or down."
And it added: "The Prince's Trust Macquarie Youth index found that over 750,000 young people believe they have nothing to live for, with jobless youngsters at risk of poor mental health."
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