While staying active boosts wellbeing, exercising for over three hours a day may indicate obsessive characteristics and worsen one's mental health, a lancet study has found.
The study of 1.2 million people in the US has found that people who exercise report having 1.5 fewer days of poor mental health a month, compared to people who do not exercise.
Small reductions were still seen for people who exercised more than 90 minutes a day, but exercising for more than three hours a day was associated with worse mental health than not exercising at all.
"Previously, people have believed that the more exercise you do, the better your mental health, but our study suggests that this is not the case," said Adam Chekroud, an assistant professor at Yale University in the US.
"Doing exercise more than 23 times a month, or exercising for longer than 90 minute sessions is associated with worse mental health," said Chekroud.
The study, published in The lancet Psychiatry journal, found that exercising for 45 minutes three to five times a week was associated with the biggest benefits.
The study found that team sports, cycling, aerobics and going to the gym are associated with the biggest reductions in poor mental health.
Previous research into the effect of exercise on mental health has conflicting results.
While some evidence suggests that exercise may improve mental health, the relationship could go both ways - for example inactivity could be a symptom of and contributor to poor mental health, and being active could be a sign of or contribute to resilience.
Researchers said that their study cannot confirm cause and effect.
"Depression is the leading cause of disability worldwide, and there is an urgent need to find ways to improve mental health through population health campaigns," said Chekroud.
In the study, researchers used data from 1.2 million adults across all 50 US states who completed a survey in 2011, 2013, and 2015.
On average, participants experienced 3.4 days of poor mental health each month.
Compared to people who reported doing no exercise, people who exercised reported 1.5 fewer days of poor mental health each month - a reduction of 43.2 per cent.
The reduction in number of poor mental health days was larger for people who had previously been diagnosed with depression, where exercise was associated with 3.75 fewer days of poor mental health compared with people who did not exercise - equivalent to a 34.5 per cent reduction.
Even completing household chores was associated with an improvement.
Exercising for 30-60 minutes was associated with the biggest reduction in poor mental health days. However, exercising for more than three hours a day was associated with worse mental health than not exercising at all.
