A friend on mine who has Multiple Sclerosis mentioned to me that it gets much worse for him in the winter, and then gets better heading into the spring and summer. IIRC low vitamin D is believed to make MS worse. He was going to try tanning beds this winter to see if it helps (usually tanning beds are bad news due to cancer risk, but in his medical case his doctor recommended it). Disclaimer: All anecdotical.

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Vitamin D is synthesized by your skin from sunlight using UVB rays. This is interesting to note, indicating that being far from the equator does not mean you cannot get enough UVB rays from sunlight during the winter:

The assumption that vitamin D levels in the population follow a latitude gradient is especially questionable in view of surveys which have shown that UVB penetrating to the earth's surface over 24 hours during the summer months in northern Canada equals or exceeds UVB penetration at the equator. Accordingly, there is sufficient opportunity during the spring, summer, and fall months at high latitude for humans to form and store vitamin D3.

- http://en.wikipedia.org/wiki/Vitamin_D#Production_in_the_ski... - http://www.nap.edu/openbook.php?record_id=13050&page=104

Perhaps if true, that means when it's cold out people stay indoors and wear lots of clothing when outside, effectively limiting sunlight exposure, meaning less vitamin D production in the winter...