I'm very sensitive to caffeine and I have come up with one rock-solid rule that works for me: i can have a cup of coffee or tea but then, on the next day, I have to be caffeine-free, no exceptions, ever. The way I explain it to myself is that my body goes into an unnatural mode of operation on caffeine, the mode that is both productive and creative - and even pleasant, but I need some time to go back to normal state, which takes at least a day.

If I have a low caffeine drink like a lightly brewed green tea, I don't get much of the benefit but still get the downside: I need to completely cut off caffeine for a time.