It’s a widely-held belief that exercise improves mental health, and a new study of 1.2 million Americans has added significant weight to the theory.

However, the researchers also found that exercising too much actually has a detrimental effect on one’s mental wellness.

The study, conducted by Yale University and the University of Oxford and published in The Lancet Psychiatry Journal, concluded that people who exercise report having 1.5 fewer days of poor mental health a month, compared to people who do not exercise at all.

While all types of activity appeared to improve mental health, the most effective ones were found to be team sports, cycling, aerobics and going to the gym, according to the largest observational study of its kind.

The researchers drew their conclusions by analysing data from 1.2 million adults across all 50 US states who completed the Behavioural Risk Factor Surveillance System survey in 2011, 2013 and 2015.

The data took into account demographics as well as information about participants' physical health, mental health and health behaviours, but aside from depression, mental health disorders were not included.

Participants were asked to estimate how many days in the past 30 days they would rate their mental health as “not good” based on stress, depression and emotional problems. They were also asked how often they exercised in the past 30 days outside of their regular job, as well as how many times a week or month they did this exercise and for how long.

The researchers took into account age, race, gender, marital status, income, education level, employment status, BMI, self-reported physical health and previous diagnosis of depression.

Researchers concluded that 45 minutes of exercise three to five times a week was the optimum amount of time for the greatest benefits to mental health.

They found that the average person experienced 3.4 days of poor mental health each month.

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However, 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.

Although certain activities appeared to have a greater positive effect than others, even completing household chores was associated with a reduction in poor mental health days of around 10 per cent.

While previous studies have suggested links between staying active and improving mental health - earlier this year, for example, research by the University of Limerick found lifting weights is associated “with a significant reduction in depressive symptoms” - it is still somewhat unclear whether inactivity is a symptom of or contributor to poor mental health. The authors of the new research note 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,” says Dr Adam Chekroud, Assistant Professor of Psychiatry at Yale University, and Chief Scientist at Spring Health, USA.

“Exercise is associated with a lower mental health burden across people no matter their age, race, gender, household income and education level.

“Excitingly, the specifics of the regime, like the type, duration, and frequency, played an important role in this association. We are now using this to try and personalise exercise recommendations, and match people with a specific exercise regime that helps improve their mental health.”

But how often and how much you exercise plays a role too - exercising more or less than 30-60 minutes between three and five times a week decreases the benefits of exercising on your mental health.

Working out for more than 90 minutes a day was still linked to reduced number of mental health days than not exercising at all, but hitting the gym for more than three hours a day was associated with worse mental health than not exercising at all.

The authors suggest that people doing extreme amounts of exercise might have obsessive characteristics which could place them at greater risk of poor mental health.

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“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. Doing exercise more than 23 times a month, or exercising for longer than 90 minute sessions is associated with worse mental health,” continues Dr Chekroud.

“Our finding that team sports are associated with the lowest mental health burden may indicate that social activities promote resilience and reduce depression by reducing social withdrawal and isolation, giving social sports an edge over other kinds.”

Dr Gary Cooney, Gartnavel Royal Hospital, Glasgow, comments: “There is gathering interest and momentum around research into exercise as a treatment for mental health disorders.

“The appeal is multifaceted: patients, particularly those reluctant to pursue medication or psychological approaches, are drawn to the self efficacy of exercise, the ability to attain a degree of agency in their own process of recovery.