Caffeine doesn't appear to have an effect on me either. When I stop consuming caffeine I don't normally notice an effect until the next morning, when I'm usually a lot worse at getting up in the morning, but once I'm up I don't notice a thing.

I've gone months without caffeinated drinks before and I've never noticed withdrawal effects. Although this may be because I don't notice any effects of caffeine when I do drink it. Unless I'm drinking an energy drink like Rockstar, I never get the 'pick me up' effect people claim from a coffee.

I wonder if, just like some people are caffeine sensitive, if some people are simply caffeine insensitive and don't notice the effects one way or another.