Using social media isn't directly harming teenagers - but it can reduce the time they spend on healthy activities, such as sleeping and exercising, a study suggests.
Parents should ban phones from bedrooms after 22:00 and encourage more physical activity, the UK researchers said.
Girls were particularly vulnerable to cyber-bullying on social media, which could lead to psychological distress.
But what drove boys' distress needed more research, the study said.
In the UK, nine out of 10 teenagers use social media and there is growing concern about its impact on the mental health and the wellbeing of young people.
So far, research has thrown up contradictory evidence because of the lack of long-term data.