Can working a third shift job affect your mood, similar to seasonal depression?

Yes: Working at night puts you at odds with your natural circadian rhythm, causing physiological and emotional stress and fatigue. Here's an APA article summarizing the effects: http://www.apa.org/monitor/2011/01/night-work.aspx Researchers have found that intermittent bright light during your night shifts, wearing sunglasses on the way home, and sleeping in a very dark room can help this a lot.