A Surprising Link between ADHD and Exposure to Sunlight
Although ADHD is considered a neurobiological disorder in which genes are an important risk factor, environmental factors also contribute to its development. Sometimes, the links to environmental factors can be extremely interesting, not to mention surprising. For example, several studies reviewed in Attention Research Update suggest that exposure to natural, green outdoor environments are associated with a reduction in ADHD symptoms, at least temporarily.
You can find a summary of this work at http://www.helpforadd.com/2009/february.htm
Recently, I came across an interesting article in Biological Psychiatry which suggests that living in states with greater sunshine - the technical term is solar intensity (SI) - may protect against the development of ADHD [ Arns et al., (2013). Geographic variation in the prevalence of Attention-Deficit Hyperactivity Disorder: The sunny perspective. Biological Psychiatry, http://dx.doi.org/10.1016/j.biopsych.2013.02.010]
Data for this study came from the 2003 and 2007 National Survey of Children's Health in children under 18. Both data sets included nationally representative samples involving thousands of children across the U.S. As part of the survey, parents were asked whether a doctor or other health care provider had ever told them that their child had 'attention deficit disorder or attention deficit hyperactivity disorder". This information was used calculate the prevalence of ADHD in each of 49 states.
There was wide variation in prevalence across states - in the 2007 data, this ranged from a low of 5.6% in Nevada to a high of 15.6% in North Carolina. This large discrepancy by state is difficult to understand. Genes play a role in the development of ADHD but it seems highly implausible that genetic variation by state could explain this degree of variance. There may also be state/regional differences in diagnostic practices. However, physicians and mental health professionals are supposed to apply the same criteria wherever they practice and this also seems unlikely to account for such large discrepancies across states.
What might be an important contributing factor? The authors hypothesized that state level differences in solar intensity (SI), i.e., defined as the average amount of sunlight received each year, was a possibility. As I understand it, the rationale for this hypothesis is as follows: Many individuals with ADHD have sleep difficulties and sleep problems also exacerbate attention difficulties. Bright sunlight helps regulate our circadian rhythms, thus enhancing the quality of sleep and contributing to better daytime alertness. Thus, living in states with greater SI could protect against the development of ADHD.
To test this hypothesis, the researchers examined the association between ADHD prevalence in each state with each state's solar intensity rating; the latter was obtained from the U.S. National Renewable Energy Laboratory. They controlled for a variety of factors that could also differ by state - and possibly be linked to the prevalence of ADHD - including Medicaid coverage, male/female ratio, racial/ethnic differences, and altitude. Even after controlling for factors, more than one-third of the variation in ADHD prevalence by state was explained by variation in solar intensity.
It is interesting to note that the relationship between solar intensity and ADHD prevalence was non-linear; in other words, the relationship was not consistent across all levels of solar intensity. Instead, in states where solar intensity was the highest, i.e., Arizona, Nevada, California, Utah, and Colorado, ADHD prevalence was the lowest. However, once solar intensity dropped below the highest level, its relationship with ADHD prevalence was more modest.
To see whether this association was unique to ADHD, the researchers also tested for a link between solar intensity and state level differences in child depression and autism spectrum disorders . No relationship was found.
As a further test of their hypothesis, the authors examined the association between solar intensity and the prevalence of ADHD across 9 countries. In this study, participants were interviewed in person and were retrospectively assessed for childhood ADHD using a structured psychiatric interview. Results indicated that over 50% of the variation in prevalence across countries was related to variation in solar intensity. As with the U.S. results, prevalence was lowest in countries where solar intensity was the highest.
Summary and Implications
Results from this interesting study provide strong suggestive evidence that exposure to high levels of sunlight protects against the development of ADHD. The results were consistent with high solar exposure being a protective factor as opposed to very low solar exposure being a risk factor. This was evident in the fact that states with the highest solar intensity had the lowest rates of ADHD while states with the lowest solar intensity did not necessarily have the highest rates.
Why might this be the case? An interesting suggestion made by the authors draws on recent findings that "...increased use of modern media (IPads, mobile phones) by children and adolescents, especially shortly before bedtime, results in delayed sleep onset, shorter sleep duration, and melatonin suppression". Increased exposure to these devices, and the particular wavelengths of light they produce, may disrupt natural circadian rhythms. They go on to speculate that the apparent preventative effect of high solar intensity on ADHD might "...result from the ability of intense natural light during the morning to counteract the phase delaying affects of modern media in the evening, thus preventing delayed sleep onset and reduced sleep duration."
Because this was a non-experimental study, there is no way to conclude with certainty that being raised in environments with high levels of natural sunlight protects against the development of ADHD. The authors acknowledge this and highlight the need for additional controlled studies of this issue. One interesting study they suggest would be to determine whether deliberately exposing children to more natural light during the day, e.g., skylights to increase the natural light in classrooms and providing more outside play time in the morning when solar intensity is stronger, reduces the number of children who develop ADHD and/or reduces the intensity of symptoms in children with the disorder.
Hopefully, this type of interesting approach will be evaluated soon. In the meantime, this interesting study highlights the value of being open to - and systematically investigating - new ideas about factors that may contribute to the development of ADHD. Such work has the potential to increase our understanding of ADHD and to suggest novel ways to treat or perhaps even prevents its development.
David Rabiner, Ph.D.
Dept. of Psychology & Neuroscience
Durham, NC 27708