Its not nesesery. Take some extra vit. D in winter. Very helpful are antidepressive lamps i bought one and it is great.
I already take vitamin D during the winter. But it seems that it doesn't help. Those lamps are very expensive and I am not sure it will work.
Go to one of the tanning booths, maybe a couple of times per month. It helps process the vitamine D.
Not necessary to get a tan, just absorb a few rays. You'll feel more energetic, less sluggish.