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
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
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 Brain
clinics, in the department of experimental psychology at Utrecht University in
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% to 8%. In the darkest states, which included a
swath of the Northeast, rates of ADHD ranged from 10% to 14%.
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.
The role of vitamin D
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
Arns said about 80% 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.
Body clocks on track
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 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 behavioural paediatrics at the Steven &
Alexandra Cohen Children's Medical Centre in New Hyde Park, New York.
"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
Visit the US Centres for Disease
Control and Prevention for more on ADHD.