STAND UP!

This is huge and I'm not even going to lie. The excerpts quoted below are taken directly from a WebMD article that can be found in its entirety here. They say this better than I could.
http://www.webmd.com/fitness-exercise/news/20100119/prolonged-sitting-boosts-bad-health

That being said, a new study just came out from the British Journal of Sports Medicine. It suggests "that people who sit still for prolonged periods of time -- such as desk workers or couch potatoes -- have a higher risk of disease than those who move a muscle every now and then in a non-exercise manner."

There's a new definition for sedentary lifestyle we have to consider.
"Prolonged sitting promotes a lack of whole-body muscle movement, which the Swedish-based researchers say is the more correct way to define sedentary behavior."

Even if you exercise, you may be leading a sedentary lifestyle. "... the research team proposes that sedentary behavior is instead a distinct class of behaviors, unrelated to a lack of exercise, that boost bad health. Behaviors can include habits like TV watching...A woman’s risk of metabolic syndrome, precursor to diabetes and heart disease, jumps 26% for every extra hour she sits in front of the TV." "Whole-body muscular inactivity associated with prolonged sitting has also been strongly linked to obesity and even certain types of cancer."

Dude, kick that office chair out and trade in that desk for a tall drafting table.