Association between ADHD and intensity of sunlight

Association between ADHD and intensity of sunlight: Can ADHD be prevented?

Nijmegen, October 21st, 2013 – A study published today in Biological Psychiatry (Do Sunny Climates Reduce ADHD?) sheds new light on the increasing rates (prevalence) of attention-deficit/hyperactivity disorder, known as ADHD. Children with ADHD have problems with inattention, distractibility, disorganization, impulsiveness, and overactivity. This study found that “sunny” regions with high solar intensity, such as the US states of California, Arizona, and Colorado, and countries like Spain and Mexico have lower prevalence of ADHD. An apparent protective effect of sunlight accounted for 34-57% of the variance in ADHD prevalence. The authors speculate that this may be related to sunlight’s effects on preventing circadian rhythm (“biological clock”) disturbances. These results suggest ways to prevent or treat ADHD for a substantial sub-group of patients. 

Researchers from Utrecht University, Research Institute Brainclinics, Leiden University, and Ohio State University today published their work suggesting a possible preventive effect of sunlight on ADHD. Examining ADHD prevalence rates per US state (CDC data, see figure 1a) and solar intensity maps (NREL data, see figure 1b) the authors observed a striking geographical coincidence between low ADHD prevalence and high solar intensity. Solar intensity is a measure of how much sunlight a specific area receives (in kWh/m2/day), often used for calculating how much energy solar panels will generate.

Figure 1a: ADHD Prevalence (CDC, 2003)                            Figure 1b: Solar-Intensity (NREL)

Based on this initial observation, the researchers further investigated this in 2 US and one non-US datasets, where they found a clear negative dose-response effect between solar intensity and the prevalence of ADHD. Considering all data, solar intensity accounted for 34-57% of the variation in ADHD prevalence. Many potential confounding factors were adjusted for in their analysis such as socio-economic status, low birth weight, infant mortality, geographical latitude, ethnicity/race, etc. However, these factors could not explain the results.

ADHD and sleep

Many patients with ADHD suffer from sleep problems, most often a difficulty falling asleep (or sleep-onset insomnia). For more information also see Sleep and ADHD. Shorter sleep duration and sleep restriction are associated with attention problems. The authors suggest that sleep disturbances may explain the attention problems in this subgroup of ADHD patients. Difficulty falling asleep has been linked to circadian rhythm problems (disturbances of the biological clock) and a delayed melatonin response. It is well known that sunlight is the strongest influence on the human biological clock. The authors hypothesize that this delayed circadian rhythm and difficulty falling asleep may be caused by increased evening use of modern media, such as tablet computers and smart phones, especially with social media increasing the exposure to such blue-light* sources during the evening. The authors speculate that strong sunlight during the day might reset the biological clock and act as an antidote to the evening exposure to artificial blue light causing sleep onset problems. The implications of these findings are that future research in ADHD should take into account sleep and circadian issues. 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 (e.g. f.lux software). Finally, these results could 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.

* It is known that only 464-484 nm blue light specifically affects the biological clock, incandescent lamps have a low proportion of light in this range, whereas screens and some LED lights have a higher proportion in this range.

Reference: Arns, M. van der Heijden, K.B., Arnold, L.E. & Kenemans, J.L. (2013) Geographic variation in the prevalence of ADHD: The Sunny perspective. Biological Psychiatry DOI: 10.1016/j.biopsych.2013.02.010