A team led by Brian A Primack, MD, EdM, MS, of the University Of Pittsburgh School Of Medicine analysed the data from 4,142 teens in National Longitudinal Survey of Adolescent Health (Add Health).

The participants were not depressed at the beginning of the study in 1995. The teens were asked how many hours they had spent during the last week watching television or video cassettes, playing computer games or listening to the radio (the survey was conducted before DVDs or the Internet became widely used).

They reported an average of 5.68 hours of media exposure per day, including 2.3 hours of television, 0.62 hours of videocassettes, 0.41 hours of computer games and 2.34 hours of radio.

Over the period of seven years participants were screened and 308 had developed symptoms consistent with depression

"In the fully adjusted models, participants had significantly greater odds of developing depression by follow-up for each hour of daily television viewed," the authors wrote.

"In addition, those reporting higher total media exposure had significantly greater odds of developing depression for each additional hour of daily use," they added.

Moreover, given the same amount of media exposure, young women were less likely to develop symptoms of depression than young men.

The researchers suggest that media exposure could influence the development of depression symptoms through many different mechanisms.

They said the time spent engaging with electronic media may replace time that would otherwise be spent on social, intellectual or athletic activities that may protect against depression.

Moreover, media exposure at night may disrupt sleep, which is important for normal cognitive and emotional development.

The messages transmitted through the media may reinforce aggression and other risky behaviours, interfere with identity development or inspire fear and anxiety.