Low Vitamin D levels is associated with Seasonal Affective Disorder (SAD). It comes from lack of sunlight. Hence it's felt worse by people who don't work near windows and those that live north enough that the days grow shorter during winter time.

One of the best things you can do to raise your D levels and combat SAD is to use a therapy lamp with atleast 10,000units. It just takes a 1/2hr a day. It improves vitamin D levels, reduces depression, aids in an improved sleep, pattern amongst other positive effects.

I have taken D but have found immediate improvement with my lamp.