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Scientists don't know what causes ADHD. What they do know is genes play a role. Prenatal exposure to tobacco and alcohol as well as environmental toxins like lead are also associated with hyperactivity that is associated with impulsiveness, inability to pay attention and delayed brain maturation in children.
Other causes that are linked to ADHD include premature birth and low birth-weight.
Other things scientists know is that the incidence of ADHD varies by region.
Data maps released by the U.S. Centers for Disease Control and Prevention and the U.S. Department of Energy show ADHD prevalence rates by states. Those with more sunshine have fewer children with ADHD.
Dr. Martijn Arns and his colleagues at Southwestern University who conducted the study also know ADHD has been successfully treated with light therapy that restores normal Circadian rhythm.
For their research, the investigators analyzed multiple data-sets that they collected from the U.S. in addition to 9 other countries.
The results showed a lower prevalence of ADHD in areas with higher solar intensity even after adjusting for other known factors that contribute to the neurological disorder.
To ensure the finding was valid, the researchers also looked at prevalence of autism and depression, finding a sunny climate had no impact on numbers of cases of either.
"The reported association is intriguing, but it raises many questions that have no answers," commented Dr. John Krystal, Editor of Biological Psychiatry. "Do sunny climates reduce the severity or prevalence of ADHD and if so, how? Do people prone to develop ADHD tend to move away from sunny climates and if so, why?"
One suggestion Dr. Arns said in a press release is that "...manufacturers of tablets, smartphones and PCs could investigate the possibility of time-modulated color-adjustment of screens, to prevent unwanted exposure to blue light in the evening," which might be beneficial from a public health perspective.
Arns also suggests exploring whether skylights in classrooms might be helpful as well as scheduling children's play time in line with their biological clock.
The study suggests living in a sunny climate might be beneficial for curbing the incidence of ADHD and possibly treating the disorder. But parents should stay put until more studies are done to prove it's really so.
"Geographic Variation in the Prevalence of Attention-Deficit/Hyperactivity Disorder: The Sunny Perspective" by Martijn Arns, Kristiaan B. van der Heijden, L. Eugene Arnold, and J. Leon Kenemans (doi: 10.1016/j.biopsych.2013.02.010)