My caffeine tolerance is pretty low too. If I have any coffee after about 2 or 3 in the afternoon, I'll be up all night. Caffeine is potent stuff. Luckily it's not super addictive. It's half life in the body is something like 3 to 5 hours but can take much longer in some people. I think I'm one of those people. Some folks can drink coffee just before bed and not be affected. If I tried that, I'd be lying in bed staring at the ceiling and grinding my teeth like a meth head.