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Using social media isn't directly harming teenagers - but it can cut down the time they spend doing healthy things like sleeping and exercising.

In the UK, nine out of 10 teenagers have an social media profile and many people are concerned about the impact it can have on mental health.

That's according to research published in the scientific magazine, the Lancet Child & Adolescent Health journal, where scientists spoke to 12,000 kids aged between 13 and 16 in schools in England.

The study suggests that children be encouraged to take part in more physical activity and ban mobile phones in kids bedrooms after 10pm.

In the study, teenagers in Year 9 were asked how often they checked their social-media accounts on Instagram, Facebook, Whatsapp and Twitter each day.

More than half of the girls asked and 43% of the boys used social media more than three times times a day.

However, those numbers got bigger as kids get older, rising to 69% of boys and 75% of girls by Year 11.

The research found that boys and girls who checked their social-media more than three times a day had poorer mental health.

The study also found girls were particularly vulnerable to cyber-bullying and, as they get older, are more likely to become anxious, but boys were not.

Dr Dasha Nicholls, from Imperial College London, worked on the study and says

"In cyber-bullying, even your bed is not a safe place. And if your phone is downstairs, you can't be bullied in your bed."

Dr Nicholls added that social media isn't always bad, but it can get in the way of other important things like exercise and sleep.

"It's not the amount of social media... it's when it displaces real life contact and activities.

"It's about getting a balance."