Vitamin D is probably a factor, but probably not a major factor for me because my symptoms turn on and off, whereas one would expect them to gradually increase or diminish as Vitamin D levels rose and fell over the seasons.

I have done a lot of reading on this fascinating topic, and just recently came across a really neat app for my smartphone called dminder. It lets you input your specifics like age, weight, skin tone, dietary habits, etc, and then it calculates your vitamin D intake by taking into account your location, time of day, exposure, clouds, etc. I tried it out today, and even on the southern AZ border (lat 32*) one can only synthesize Vitamin D during the middle 4 hours of the day because the max angle of the sun reaches only 41* now. I went for a hike, and it says that I picked up something like 8,000 Units of the Sunshine Vitamin.