First things first, do you get enough sleep at night?
I work a desk job and do consistently get pretty drowsy around 2pm. I find that eating a lighter lunch and moving around keeps me awake. Take a walk around the building or something, it's bad for you to sit all day long anyways. I drink an energy drink every day as soon as I get in but I don't know what effect, if any, it has on my afternoon drowsiness.