Attention-deficit/hyperactivity disorder (ADHD) is characterized by an inability to focus, poor attention, hyperactivity, and impulsive behavior, and a delay in the process of brain maturation. Many individuals with ADHD also report sleep-related difficulties and disorders. In fact, sleep disorder treatments and chronobiological interventions intended to restore normal circadian rhythms, including light exposure therapy, have been shown to improve ADHD symptoms.

Estimates suggest that the average worldwide prevalence of ADHD ranges from about 5% to 7%, but it also varies greatly by region. A visual comparison of data maps released by the US Centers for Disease Control and Prevention and the US Department of Energy that display ADHD prevalence rates by state and solar intensities across the country, respectively, reveals an interesting pattern indicative of an association. So does this mean that there could be an identifiable relationship between ADHD prevalence rates and the sunlight intensity levels of particular regions?

The accumulation of these points led Dr Martijn Arns and his colleagues to investigate. They collected and analyzed multiple data-sets from the United States and nine other countries. Reporting their findings in Biological Psychiatry, they did find a relationship between solar intensity and ADHD prevalence.

Even after controlling for factors that are known to be associated with ADHD, both US and non-US regions with high sunlight intensity have a lower prevalence of ADHD, suggesting that high sunlight intensity may exert a protective effect for ADHD.

To further validate their work, they also looked at this same relationship with autism and major depressive disorder diagnoses. They found that the findings were specific to ADHD, with no associations observed between the other two disorders.

“From the public health perspective, 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,” Arns says in a release. “These results could also point the way to prevention of a sub-group of ADHD, by increasing the exposure to natural light during the day in countries and states with low solar intensity. For example, skylight systems in classrooms and scheduling playtime in line with the biological clock could be explored further.”