I drink a lot of caffeine. I have a really strong 16oz cup, in the morning, that I think is around 250mg of caffeine and, every afternoon, I follow that up with a 300mg energy drink. I sip both of these very slowly over a couple of hours (usually my coffee, with heavy cream, is cold by the time I'm done with it). On random days I'll even have a little more than that. Caffeine increases dopamine and, for me, it's a mood lifter and can make me feel better when I'm in a funk. Obviously, for others, the opposite can happen when it causes jitters and ratchets up your anxiety.
I do secular weddings (4 so far, over the past 6 years, and only for very close friends) and I recently found that getting loaded up on caffeine really helped me, believe it or not, to really relax, calm down and flawlessly perform the ceremony in front of 200 people. The other ceremonies I did went well too but I absolutely nailed that last one. I had the least amount of "nerves" than ever before. Caffeine is now my secret weapon if I am ever asked to do another one.
Now. . .having said all that I want to get off of caffeine. Besides dopamine it also increases the release of cortisol, the stress hormone. This is not a good thing and is probably why some get extreme anxiety on it. Now that I am sober and eating healthy I'd like to, eventually, see what could happen if I went completely clean. Besides anxiety, some people find that a lot of muscle tension goes away when they stop caffeine. They also report their energy levels are more consistent across their days. I've read that one needs to quit for at least two months to realize maximum results.