By Brenda Goodman
TUESDAY, Oct. 29 (HealthDay News) -- Sunny days can be a big distraction for those who are tethered to their desks, but a new study suggests that sunlight may actually lower the prevalence of attention-deficit/hyperactivity disorder (ADHD).
Scientists mapped the number of ADHD diagnoses across the United States and in nine other countries. They compared those rates to the intensity of sunlight those regions receive year-round.
Regions that got the most sun had rates of ADHD diagnoses that were about half as high as regions that got the least, according to the research.
"The maps line up almost perfectly," said study author Martijn Arns, director of Brainclinics, in the department of experimental psychology at Utrecht University in Nijmegen, Netherlands.
In the United States, the sunniest states were in the Southwest and West and included Arizona, California, Colorado, Nevada, New Mexico and Utah. Rates of ADHD diagnoses in those states ranged from 6 percent to 8 percent. In the darkest states, which included a swath of the Northeast, rates of ADHD ranged from 10 percent to 14 percent.
The relationship between ADHD and sunlight held steady even after researchers adjusted their data to control for other factors that might account for differing rates of ADHD diagnoses, such as race, poverty and the male-to-female ratio in each area.
Researchers even considered whether vitamin D, which is produced in the body after exposure to sunlight, might account for the differences, but they said a prior study ruled that out.
They also examined whether more sunlight might be tied to lower rates of other kinds of mental disorders, including depression and autism. It wasn't.
The researchers admitted that the link could just be a coincidence, and there isn't necessarily a cause-and-effect relationship between sunny climates and lower rates of ADHD diagnosis. But since some children and adults with ADHD have disrupted body clocks, which are regulated by light, they believe the relationship deserves further investigation.
Arns said about 80 percent of adults and about one-third of children with ADHD have trouble falling asleep at night. Some studies have found that these night-owl tendencies are driven by a delayed peak in the sleep hormone melatonin.
Melatonin seems to be especially disrupted by the blue wavelengths of visible light, Arns said. Energy-saving LED light bulbs, as well as the screens of tablets, smartphones and computers emit blue light. When people use those devices in the evening, it can delay melatonin release and disrupt sleep.
But Arns said people who live in sunny climates may get some natural protection from this sleep upset because they get a healthy dose of bright light in the morning, which keeps their body clocks on track.
He's currently exploring ways to test his theory.
An expert who was not involved in the study, which was published in the Oct. 15 issue of the journal Biological Psychiatry, said he's not sure melatonin is the best explanation.
Children in sunny climates may spend more time playing outside, for example, said Dr. Andrew Adesman, chief of developmental and behavioral pediatrics at the Steven & Alexandra Cohen Children's Medical Center in New Hyde Park, N.Y.
"There's a small but growing literature talking about exercise as a way to moderate ADHD and hyperactivity," Adesman said. "There could be other variables that are responsible."