There was an interesting study (sorry I don't like to lookup citations on my mobile) showing that the prognosis for skin cancer patients is better in the summer than the winter, presumably because they get more sunlight. Not causative, but interesting nonetheless.
Also MS is extremely prevalent at high latitudes compared to the tropics. Why? We know vitamin D has a role in protecting the insulation of the nerves, so maybe that's the mechanism. Anecdotal, but the one person I know with MS is white as the snow and avoids the sun like it's the devil.
This is an extremely popular simplification. The scientific jury is still out on this one, as it seems that paradoxically people with constant exposure to sunlight are less likely to get skin cancer as compared to those that get sporadic exposure.