The incidence of ADHD may be lower in regions with a sunny climate, new research indicates.

ADHD is a behavioural disorder, the main symptoms of which include inattention, impulsivity and hyperactivity. It can lead to disruptive behaviour in the classroom and as a result, is often first identified in school. Up to one in 20 Irish children has the condition and it can continue into adulthood.

It is still unknown what exactly causes the condition. While genetics have a role, other risk factors that have been linked to it include a premature birth, a low birth weight and a mother smoking during pregnancy.

According to US and Dutch scientists, the average worldwide prevalence of ADHD is between 5-7%, however this can vary a lot depending on the region. A series of data maps released by the US Centers for Disease Control and Prevention revealed that those regions in the US with the highest levels of solar intensity (sunlight) had the lowest prevalence of the condition.

The scientists decided to look into this further. They collected and analysed data from the US and nine other countries and found that solar intensity did appear to be linked to the prevalence of ADHD.

Even after taking other factors into account, throughout the 10 countries analysed, those regions with the highest levels of sunlight had a lower prevalence of ADHD. This, the scientists said, suggests that sunlight might offer some sort of ‘protective' effect against the condition.

The scientists carried out the same analyses in relation to autism and major depressive disorder and found that their findings were specific to ADHD.

While acknowledging that this study displays a link and not an actual cause, they insisted that these findings are important.

"These results could point the way to prevention of a sub-group of ADHD, by increasing the exposure to natural light during the day in countries with low solar intensity. For example, skylight systems in classrooms and scheduling playtime in line with the biological clock could be explored further," the team said.

Details of these findings are published in the journal, Biological Psychiatry.