But the maximum benefit seems to be obtained from 14 hours of sport a week, which is double the official recommendation of seven hours for this age group, the study shows.

The researchers quizzed more than 1,200 sixteen to twenty year olds in the French speaking part of Switzerland between February 2009 and January 2010 about how much sport they did.

Their mental and physical wellbeing was assessed using validated scoring criteria from the World Health Organization, on a scale of 0 to 25. A score below 13 indicates poor wellbeing.

Half the sample was male, with an average age of just under 18. Just under one in 10 (9%) was overweight or obese. The average wellbeing score for the entire sample was 17.

Weekly sports participation was categorised as low (0-3.5 hours; 35% of respondents); average (3.6-10.5 hours; 41.5%); high (10.6-17.5 hours; 18.5%); and very high (more than 17.5 hours; 5%).

Compared with the teens in the average group, teens in the low and very high groups were more than twice as likely to score below 13 on the WHO wellbeing scale, corresponding to an inverted U shaped association between weekly duration of sport practice and wellbeing.

Those in the high group, on the other hand, were around 50% less likely to score below 13.

The peak scores of wellbeing obtained were for around 14 hours a week of sports practice a week -- double the amount recommended for this age group -- with this protective effect reversed beyond 17.5 hours a week.

Regular exercise is known to have a positive impact on mental and physical wellbeing, reducing stress and anxiety, and boosting self-esteem and brain power, say the authors.

But while doubling the recommended weekly time spent playing sports to 14 hours seems to be good for mental and physical health at this age, going beyond this seems to be detrimental and ceases to be protective, they conclude.