Teenagers who spend hours on social media could be risking depression.
Experts said the internet encouraged youngsters to measure themselves against others, usually those with 'perfect' bodies and exciting lifestyles.
This 'upward social comparison' can significantly increase the symptoms of depression and low self-esteem.
Social media also affects those who are already feeling low by putting them in a 'reinforcing spiral', an academic study suggests.
Researchers tracked 3,826 teenagers over four years, monitoring their use of TV, internet, video games and social media. The participants, who were aged 12 at the start, had their emotional health measured on a 28-point scale.