Not only that, but a large fraction of the English-speaking northern hemisphere does not receive enough sunlight in winter months for the skin to produce Vitamin D.
I often arrive at our offices in the dark, and then leave after dark. In the meantime I'm in a windowed corner office but like others have said - filtered light. I take 10k IU's of vitamin D per day. Absent that, my natural (un-supplemented) vitamin D tests come back with a level about 20+ (30-100 being the desired range).