I doubt whether students know (or care) that exam day can be almost as stressful for the professor as for the student. (Well, not quite, but we're older and more decrepit.) You have to be in the building, just in case some problem with the exam arises. If you run into students before the exam, a large percent give you the evil eye (sort of the way suspected heretics might have watched Torquemada). Even worse, if you run into them afterwards, a certain percentage start eyeing you the way Tony Soprano does when he's sizing up a guy for cement overshoes. Being the delicate type that I am, I tend to make a mad dash for my office, hunker down, and keep my fingers crossed.