Caffeine never used to bother me but I had to quit it completely due to bone issues as my doctor said it can worsen bone loss in women. She said to picture the caffeine molecules carrying away tiny calcium molecules. Yes, over simplified but at times I need that to prevent getting overloaded with info and forgetting it all these days (I sometimes do better with pictures in my mind then words, she was an awesome doctor who always tried to make sure i understood the whys lol)
But I too found out at that time that maybe I had just got so used to it I didn't know it was affecting me as I could drink a pot of coffee and sleep, unless insomnia hit. Now that I was off it for over ten years, if I have more than a cup, I get jittery and shake and it's not a good feeling at all. I became a tisane drinker, so I still got that morning cup without the caffeine.
I did drink a lot when younger because everyone I knew did as well, I ALWAYS was a sick mess the next day. Now I just don't drink, and never once have I missed the stupid way I would get when I drank or that horrible next day. I know alcohol wasn't in the first question but since it was brought up, I figured I'd add my experience with that as well.