The study, published in the journal Lancet Psychiatry, found that an additional 60 minutes of light activity, such as walking or doing chores, daily at the age of 12 was linked with a 10 per cent reduction in depressive symptoms at age 18.
"Our findings show that young people who are inactive for large proportions of the day throughout adolescence face a greater risk of depression by age 18," said the study's lead author, Aaron Kandola from the University College London (UCL) in the UK.
"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," Kandola said in a statement.
The researchers found some associations between moderate-to-vigorous activity at earlier ages and reduced depressive symptoms. Previous studies suggests that excessive sedentary time that involves sitting and low levels of energy expenditure is related to poor mental health.
The research team used data from 4,257 adolescents, who have been participating in longitudinal research from birth as part of the University of Bristol's Children of the 90s cohort study.
The children wore accelerometers to track their movement for at least 10 hours over at least three days, at ages 12, 14 and 16.
The accelerometers reported whether the child was engaging in light activity, which could include walking or hobbies such as playing an instrument or painting, engaging in moderate-to-physical activity such as running or cycling, or if they were sedentary.
The use of accelerometers provided more reliable data than previous studies which have relied on people self-reporting their activity, which have yielded inconsistent results, the researchers said.
Depressive symptoms, such as low mood, loss of pleasure and poor concentration, were measured with a clinical questionnaire, they said.
The questionnaire measures depressive symptoms and their severity on a spectrum, rather than providing a clinical diagnosis.
Between the ages of 12 and 16, total physical activity declined across the group, the researchers said.
They found that every additional 60 minutes of sedentary behaviour per day at age 12, 14 and 16 was associated with an increase in depression score of 11.1 per cent, 8 per cent or 10.5 per cent, respectively, by age 18.
Those with consistently high amounts of time spent sedentary at all three ages had 28.2 per cent higher depression scores by age 18, the researchers found.
They caution that their data was weaker due to low levels of activity of such intensity in the group -- averaging around 20 minutes per day -- so the findings do not clarify whether moderate-to-vigorous activity is any less beneficial than light activity.
While the researchers cannot confirm that the activity levels caused changes to depressive symptoms, they accounted for potentially confounding factors such as socioeconomic status, parental history of mental health, and length of time wearing the accelerometer.
"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," Kandola said.
"The number of young people with depression also appears to be growing and our study suggests that these two trends may be linked," he added.
(With inputs from PTI)
