Geographic variation in the prevalence of attention-deficit/hyperactivity disorder: the sunny perspective

ADHD rates across 49 US states and nine other countries were compared with average sunlight intensity. Places with more sunlight had less ADHD, explaining roughly a third to over half of the variation, even after allowing for income, birth weight and latitude, and the link did not hold for autism or depression. The authors suggest better-regulated body clocks may explain it.