Hahaaaa! ... and watch as your victim backs away, slowly, with hands raised in defense... "ok, ok, sorry." Then there are the "helpful" ones who will stay, and insist you confide in them...

I usually say that I'm just a bit tired... that usually goes over well, is usually true, and and doesn't require much energy/acting. But there are the few who then want to know if I am having "trouble sleeping".

Otherwise, the most efficient answer I've found is an emphatic "yes" with a very quick but genuine smile. "Yes... thank you. " is even better. The smile is essential though. It reassures people.

Oh, the delights of social gymnastics...