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
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
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
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 social 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.
(c) 2013 David Rabiner, Ph.D.
Information presented in Attention Research Update is for informational
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