During the winter months when the days are shorter, and the skies are gray, I get depressed. I am one of those people who need sunshine to thrive. I have purchased daylight light bulbs to place in light fixtures in my office and studio, and that helps. I have also learned that surrounding myself with simple pleasures helps too, so a soothing cup of tea with vanilla added, some good music, and a colorful pen in my hand while working gives me a sense that things will get better.
My advise to you if you suffer from this malady is to surround your work area with light, and pamper yourself. Little things can make all the difference in a bad day being tolerable.