It's called Seasonal Affective Disorder. (SAD.)

The fix? Sunlight, real or faux. Faux sun is two "full-spectrum" fluorescent bulbs rated 90 CRI or higher, for a half hour or longer daily, if you can't get at least that much good strong sunlight outside or if you are sunk so deep you can't get your shoes on to go see.

Many retail stores now carry these for routine household lighting too, cheap-- used to be real hard to find and trendily expensive. We use them in our kitchen since we spend so much time in there each day, and Hardi has changed his office lights to these as well. You CAN ask your boss if they will swap out the bulbs over your cubicle as long as you provide the bulbs. I asked for this in one place repeatedly and finally the boss converted her own office to them. Her cyclical winter blues, an annual pit deeper than any I'd seen before, disappeared.

~Susan