Researchers found adolescents tended to be worse off if they played sports for only a couple of hours per week, or if they practiced close to three hours each day or more.
"Overtraining has not only a well-known impact on the body, but also on the brain and therefore on emotions, thoughts and mood," Dr. Arnaud Merglen said.
"The mechanism of low activity and very high activity is probably not the same, but the results appear quite similar," he told Reuters Health in an email.
Merglen is from the Hospital for Sick Children at the University of Toronto in Ontario and led the study, which took place at the University of Lausanne's School of Medicine in Switzerland.
The U.S. Centers for Disease Control and Prevention recommends children up to age 18 get at least one hour of exercise each day. But there hasn't been much evidence on what's best for teenagers when it comes to sports participation, according to Merglen.
He and his colleagues surveyed 1,245 Swiss adolescents age 16 to 20. They asked participants how much time they spent playing sports each week, among other things.
The participants also filled out a short survey on their well-being. Well-being was measured on a scale from zero to 25, with scores below 13 indicating poor well-being.
The researchers split young people into four groups based on their weekly sports practice: low (zero - 3.5 hours), average (3.6 - 10.5 hours), high (10.6 - 17.5 hours) and very high (more than 17.5 hours).
About 19 percent of the 438 teens that practiced the fewest hours reported not regularly feeling cheerful, relaxed and energized. Similarly, about 18 percent of the 60 teens that practiced the most had poor well-being.
That compared to about 9 percent of 517 adolescents in the average weekly sports group that had poor well-being and 4 percent of 230 young people in the high (but not very high) participation group.
"Although doubling the recommended weekly time spent practicing sports to 14 hours seems to be even better for the mental and physical health of teens, going beyond this seems to be more risky," Merglen said.
The study, published in Archives of Disease in Childhood, doesn't prove exercise caused feelings of lower well-being - it only suggests a correlation.
"It's an interesting study, and it's a good first step, but what we really need is a study where we take thousands of five-year-olds and follow them for the next 10 to 15 years and see how kids really do based on the physical activity in which they participate," Dr. Amanda Weiss Kelly told Reuters Health.
She is the division chief of pediatric sports medicine at University Hospitals Rainbow Babies and Children's Hospital of Case Western Reserve University in Cleveland, Ohio.
"Exercise is really very healthy, we want kids to exercise," said Weiss Kelly, who wasn't involved in the new study.
"Certainly the benefits of exercise outweigh the risk for injury and other issues and I think it's important for kids to exercise. We need to do our best to encourage kids, but also do it in a healthy way."
Weiss Kelly noted that sports can be stressful for kids who are elite athletes and put in long hours. That stress may, at least in part, be due to the pressures put on them by coaches and parents, rather than the hours put into practice time.
"Parents really have to listen to kids about what it is they want to do to be physically active and support them in what they choose because some of the other studies on stress and sports have demonstrated that if kids are really the ones picking what they do, and how long they do it, then they tend not to get burned out," Weiss Kelly said.
"The kids that are being pushed and feeling more pressure from parents and coaches are more likely to get burned out. I think letting kids guide the activity and be the ones in charge really helps," she said.