Recent research on the link between physical activity and depression risk in adults has suggested that exercise may offset the genetic tendency toward depression. Adults with genetic risks who exercised regularly were no more likely to develop depression than those without the genetic propensity.

There’s good evidence that this same association holds in adolescents, a group with a generally high risk of depression, and with concerningly high suicide rates. But adolescence is also a time when physical activity often becomes less common, especially among girls.

The World Health Organization recommendation is for an hour of moderate to vigorous exercise every day for adolescents, in addition to whatever they do in school — that means activity that gets you sweating and breathing hard. But many adolescents don’t exercise anywhere near that much.

So people who study adolescent mental health and suicide prevention are interested in the possible protective effects of more limited exercise or more moderate physical activity — and also, of course, in the question of how to help adolescents get moving.

A prospective study published in the March issue of the journal The Lancet Psychiatry found that even light activity — and a corresponding decrease in the amount of time that kids spent being sedentary — was linked to better mental health as they got older.

The researchers looked at the activity of adolescents at the ages of 12, 14 and 16, who were then assessed for depression at around 18. The participants wore devices called accelerometers, which continuously measured their activity during the day.

The first author on the new study, Aaron Kandola, a Ph.D. candidate in physical activity and mental health at University College London, said that many of the older studies had relied on memory, asking people what they had done in their leisure time. While this captures exercise and other moderate activity, he said, it misses light activity, which actually makes up the bulk of many people’s movement during the day — walking at a casual pace, shopping, playing an instrument, doing chores around the house.

The study found that total physical activity dropped between ages 12 and 16, mostly because of decreases in that light activity, and sedentary behavior increased. And the activity levels when kids were younger were linked to their mental health later on; the depression scores at 18 were lower for every additional 60 minutes per day of light activity at 12, 14 and 16, and higher for every additional sedentary hour.

Mr. Kandola said that by age 16, young people in the study were spending an average of close to nine hours a day on sedentary activity, and that it would be hard to decrease that behavior significantly through exercise, because it would require lots of exercise to significantly reduce that amount of sedentary time. If the goal is to reduce sedentary time significantly (say, by two hours a day), it might be useful to build in more light activity — for example, by reshaping the school environment, he said.

“We have this time at school where we have put out an agenda of sedentary behavior,” he said, citing the emphasis on sitting down, with rare breaks. “It’s time for reforms to target the school system.” He suggested that activity breaks be scheduled regularly in schools, and that schools try standing desks, which have been successful in workplaces.

Xihe Zhu, an associate professor of human movement science at Old Dominion University in Norfolk, Va., said that research shows that “some exercise is going to be much better than no exercise at all.” Dr. Zhu was the first author on a 2019 study of 35,000 children and adolescents from 6 to 17 in the United States, which found that those who reported no exercise were twice as likely to have mental health problems, particularly related to anxiety and depression, compared with those who met the exercise guidelines.

Even if children exercised only one to three days a week, he said, there was a strong correlation with lower rates of anxiety and depression — and there was no significant difference between them and those who exercised four to six days a week.

Good sleep duration and extracurricular activities were also associated with better mental health. In fact, physical activity may improve sleep quality, which is closely linked to mental health.

Elaine McMahon, a research fellow at the National Suicide Research Foundation and the School of Public Health, University College Cork in Ireland, was the lead author on a 2017 study coming out of a large research project on European adolescents.

They studied more than 11,000 13- to 15-year-olds in 10 countries, and found that only 13.6 percent of them met the recommended guidelines of 60 minutes of moderate to vigorous exercise a day.