I will try to put this in the form of a map later, but it is already clear that a definite pattern exists. The farther from the equator, the higher the PD rate. If you go through and edit out things like Florida and Alaska (high immigrant populations) or the extremes of the data set, the pattern gets even more defined. Some of you with a decent statistics background should work this over a little.

The temptation to yell "Vitamin D" is very strong here, but there is at least one other possibility to consider - circadian rhythms. I lived a short while in Alaska and the short days of winter were a real bummer. We all know of SAD and keep in mind what a problem depression is for some of us too. Insomnia can be another manifestation of circadian rhythm, as well. Don't underestimate its power as extreme disruption can kill you once it has driven you mad.

It is almost certain to be the combination of the two.