Living in geographic areas where there is intense sunlight may be linked to a lower risk for attention-deficit/hyperactivity disorder (ADHD), new research suggests.
The study showed that in sunny states with high solar intensity, such as California and Arizona, and in countries such as Spain and Mexico, the adjusted prevalence rates of ADHD were lower than in than less sunny regions. In addition, the researchers found an apparent protective effect of sunlight, which accounted for 34% to 57% of the variance in prevalence, a finding that may be related to sunlight’s effect in preventing circadian rhythm disturbances.
“Based on our previous review, we did predict this association between ADHD prevalence and solar intensity. However, we were surprised how strong this association was and that it replicated in 3 datasets,” lead author Martijn Arns, PhD, researcher in the Department of Experimental Psychology at Utrecht University in the Netherlands and director of the Research Institute Brainclinics, told Medscape Medical News.
“The #1 takeaway is that clinicians should pay more attention to sleep issues in children with ADHD and with subclinical attentional issues — and try to focus on those issues more,” said Dr. Arns.
For example, for children who have difficulty falling asleep and who have attention issues, Dr. Arns suggested that exposure to more intense early-day natural light, such as might occur by asking them to walk their dog in the morning or to walk or ride their bikes to school instead of being driven, could be beneficial.
Dr. Martijn Arns
“As a secondary step, preventing unwanted blue light exposure in the evening could be tried, though our results only implicate a preventive role of intense sunlight,” he added.
The study was published online March 25 in Biological Psychiatry.
“Remarkable” Geographic Pattern
While conducting a review study on neurofeedback in ADHD, which was published in Neuroscience and Biobehavioral Reviews in 2012, the investigators speculated about the role of sleep in ADHD and about the influence of light as a chronobiological factor, said Dr. Arns.
On the basis of that review, the investigative team inspected ADHD prevalence maps of the United States and “found a remarkable pattern” in that prevalence rates were lower in the Southwest region.
“This became much more interesting when we also compared those maps to NREL [National Renewable Energy Laboratory] maps depicting solar intensity, which resulted in a near perfect match. This led us to further investigate this association in more detail,” he said.
For the current study, the researchers examined prevalence rates of ADHD in children in 2003 and 2007, using information from the Centers for Disease Control and Prevention’s (CDC’s) National Survey of Children’s Health. These rates were then compared with NREL solar intensity maps.
“Solar intensity is a measure of how much sunlight a specific area receives (in kilowatt hours/square meters/day), often used for calculating how much energy solar panels will generate,” noted the investigators in the release.
In addition, global horizontal irradiation (GHI) data from the State University of New York were assessed for Mexico, and GHI data from the National Institute for Spatial Research were examined for Colombia. The European Institute for Energy and Transport provided data for 7 other countries.
The prevalence rates for ADHD in adults in the United States and other countries were assessed using data from the World Health Organization World Mental Health Survey Initiative. The countries included Mexico, Colombia, Belgium, France, Germany, Italy, the Netherlands, Spain, and Lebanon.
Negative Dose Response
Results showed a significant negative dose-response effect between solar intensity and the prevalence of ADHD in the 2003 (P = .0006) and 2007 (P = .0002) US child datasets.
These results were “mainly driven” by the 6 states in which the solar intensity score was 5.5 or higher: Arizona, New Mexico, Nevada, California, Colorado, and Utah.
Confounding factors that were adjusted for in the analysis included socioeconomic status, infant mortality, low birth weight, ethnicity/race, and geographic latitude. “However, these factors could not explain the results,” noted the release.
A significant negative dose response was also found between solar intensity and ADHD prevalence in adults (P = .007).
In addition, significantly lower prevalence rates of adults with ADHD were found in Spain, Mexico, Colombia, and Lebanon, and a significantly higher prevalence was found in France, compared with the rates found in the other countries that were assessed.
There were no significant associations found between levels of solar intensity and prevalence of autism spectrum disorders or major depressive disorder.
“This study is important in that it further illustrates that ADHD very likely consists of different subgroups and is not a heterogeneous disorder. A large subgroup (estimated between 50% to 80%) is characterized by a delayed circadian phase, resulting in chronic sleep restriction, which can explain the attentional issues in ADHD,” said Dr. Arns.
“Our data suggest that intense natural sunlight at the right circadian phase might at least prevent this circadian phase delay from taking place,” he noted.
The findings also suggest that researchers should be looking at sleep factors and circadian genes in ADHD and “not restricting ourselves to investigating dopamine. Theoretically, it could even be that dopamine issues in ADHD could be caused by sleep restriction and not the other way around,” said Dr. Arns.
The investigators hypothesize that increased evening use of blue-light sources, such as tablets, computers, and smart phones, may be contributing to delayed circadian rhythm and difficulty falling asleep — but that exposure to strong sunlight during the day may reset the biological clock and act as an antidote to sleep problems.
“From the public health perspective, manufacturers of [social media devices] could investigate the possibility of time-modulated color-adjustment of screens,” they conclude in the release.
Dr. Arns and 2 of the other 3 study authors report no relevant financial relationships. The other investigator reports having received travel support from Noven and research funding or advisory board honoraria from AstraZeneca, Biomarin, CureMark, Lilly, Novartis, Noven, Seaside Therapeutics, and Shire.
Biol Psychiatry. Published online March 25, 2013. Abstract