Children who spend lots of time sitting still are more likely to develop depression by the age of 18, a study suggests.
Researchers at University College London looked at the activity levels of 4,257 12- to 16-year-olds.
Those who did an additional hour of light activity each day, such as walking or chores, had fewer depressive symptoms when they reached adulthood.
The study suggests people of all ages should be encouraged to move more.
The participants, from the University of Bristol's Children of the 90s cohort study, wore accelerometers for at least 10 hours on at least three consecutive days (except when they were washing or doing water sports) at the ages of 12, 14 and 16.
These devices showed whether they were sitting still, engaging in light activity - such as walking or engaging in moderate to vigorous activity - such as running or cycling.
The children also filled out a questionnaire that measured depressive symptoms such as low mood, loss of pleasure and poor concentration.
Between the ages of 12 and 16, physical activity declined, while sedentary behaviour increased, the study says.
Researchers found the average time per day spent on:
- sitting still rose from about seven hours to eight and a half
- light activity decreased from about five and a half hours to four
- moderate to vigorous activity "remained stable"
They found for every additional hour a day spent sitting still at the ages of 12, 14 and 16, the participants' depression score rose by 11.1%, 8% and 10.7%, respectively.
While with light activity, their depression score fell by 9.6%, 7.8% and 11.1%, respectively.
By the age of 18, the questionnaire scores suggested there were 747 possible cases of depression.
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."
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."
The study, published in the Lancet Psychiatry journal, also involved King's College London and South London and Maudsley NHS Foundation Trust.
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