Scientists don't know what causes attention-deficit/hyperactivity disorder (ADHD), but a new study in the current issue of Biological Psychiatry has established a relationship between ADHD prevalence and sunlight intensity.
ADHD is characterized by an inability to focus, poor attention, hyperactivity, and impulsive behavior, and the normal process of brain maturation is delayed in children with ADHD. Risk factors that have previously been identified include premature birth, low birth weight, a mother's use of alcohol or tobacco during pregnancy, and environmental exposures to toxins like lead.
The authors of the new study note that sleep disorder treatments and chronobiological interventions intended to restore normal circadian rhythms, including light exposure therapy, have previously been shown to improve ADHD symptoms.
Initially, the researchers did a simple visual comparison of data maps that display ADHD prevalence rates by state and solar intensities (sunlight) across the country. This, they say, revealed an interesting pattern indicative of an association.
To investigate further, lead researcher Martijn Arns (Brainclinics, Netherlands) collected and analyzed multiple data-sets from the United States and nine other countries. He says that even after controlling for factors that are known to be associated with ADHD, both U.S. and non-U.S. regions with high sunlight intensity have a lower prevalence of ADHD. This, he suggests, indicates that high sunlight intensity may exert a "protective" effect for ADHD.
To validate their work, the researchers 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.
Arns cautions that the data reflects only an association - not a causation - so worried parents should not start planning cross-country moves. However, he adds, these findings do have significant implications. "From the public health perspective, manufacturers of tablets, smart-phones and PCs could investigate the possibility of time-modulated color-adjustment of screens, to prevent unwanted exposure to blue light in the evening."
The study also suggests that the results could point the way to prevention 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," Arns said.
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Source: Biological Psychiatry