A couple of thoughts come to mind:: SAD (seasonable adjustment disorder) is a winter disorder >> one of the newer findings deals with SAD and Vitamin D status. This is a very good website about vitamin D status http://www.vitamindcouncil.org/research.shtml.
Secondly you probably would benefit from taking magnesium. Calcium and magnesium are both antagonistic and synergistic. Taking high amounts of calcium will drive magnesium levels down, contrary to the general consensus magnesium is much more important than calcium. Magnesium controls over 350 enzyme reactions in the body and low levels are shown to increase heart disease, stroke, immune and neurological systems (including sleep, Depression anxiety etc.).
Vitamin D plays roles in metabolism of both calcium and magnesium. Stay away from Magnesium Oxide, try Magnesium Chloride or other good forms of magnesium.