Make sure you're getting enough sunlight. I can't overemphasize the importance of that, and of course it's so difficult in winter. If you don't have windows where you work, if at all possible get full spectrum light bulbs - they definitely help, and it's worth it even if you have to pay for them yourself. Do whatever you can to get real light, even sitting in a car or standing in an entryway on your breaks will help. If you don't have to wake up before sunrise, make sure your window shades don't block the morning sun.