So that's why I crash out in the afternoons!
I take Zoloft and haven't experienced the drowsiness. I do get up and walk around for 15-20 minutes or so, after lunch, just to keep myself active.
Do you have a desk job? If so, get away from the computer and/or desk and take a walk somewhere. Combined with the caffeine and sugar, that may help. You might also want to try stretching.