Schools could integrate light activity into the day, such as standing or active lessons – in order to stop pupils getting depressed, according to researchers.
The researchers, from University College London (UCL), found that adolescents who sit for a large proportion of the day have a greater risk of depression by the time they reach adulthood,
But those who did an additional hour of light – ie, non-strenuous – activity each day, such as walking or chores, had a reduction in depressive symptoms when they reached 18.
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Lead author and UCL psychiatry PhD student Aaron Kandola said: "We found that it's not just more intense forms of activity that are good for our mental health, but any degree of physical activity that can reduce the time we spend sitting down is likely to be beneficial.
"We should be encouraging people of all ages to move more, and to sit less, as it's good for both our physical and mental health.
"Worryingly, the amount of time that young people spend inactive has been steadily rising for years, but there has been a surprising lack of high-quality research into how this could affect mental health.
"The number of young people with depression also appears to be growing and our study suggests that these two trends may be linked."
The study, which also involved King's College London and South London and Maudsley NHS Foundation Trust, analysed data on 4,257 adolescents, who wore accelerometers to track their movement for at least 10 hours over at least three days, at ages 12, 14 and 16.
These devices showed whether the child was sedentary, engaging in light activity such as playing an instrument or moderate to physical activity such as running. They also answered a questionnaire that measured depressive symptoms such as low mood, loss of pleasure and poor concentration.
Between ages 12 and 16, sedentary behaviour rose from an average of just over seven hours to eight hours 45 minutes.
For every additional hour of sedentary behaviour per day at age 12, 14 and 16, the participants' depression score calculated from the questionnaire rose by 11.1 per cent, 8 per cent or 10.7 per cent respectively by age 18.
Those who spent consistently high amounts of time sedentary at all three ages had 28.2 per cent higher depression scores by age 18 than those who were rarely sedentary.
The study's senior author, Dr Joseph Hayes, from Camden and Islington NHS Foundation Trust, said: "Light activity could be particularly useful because it doesn't require much effort and it's easy to fit into the daily routines of most young people. Schools could integrate light activity into their pupils' days, such as with standing or active lessons."