If I drink zero cups of coffee, I feel pretty lousy. Headaches, withdrawal symptoms and the like. Once every few months I will do a caffeine reset and try to go 3-4 days without any caffeine, but I am pretty unhappy while doing it.
Yet if I drink just one cup of coffee, I feel surprisingly decent. Today I've had one cup, went to the gym, and here I am writing a blog post, and generally behaving like a normal human being.
The health/exercise benefits of caffeine are well documented. Caffeine, in a reasonable dose, is a very good thing for the body.
But the curve is U-shaped (like it is for many things we ingest). The effects of too much caffeine is also well-documented. Man, I love me some caffeine, but I don't want to destroy my nervous system either.
Each cup makes me feel a little better, but there are diminishing returns. Meanwhile, the dosage that is beneficial for me increases rapidly and falls sharply.
So my general thought for this low-caffeine morning is that you should always try to find the minimum effective dose. Know what you need as a baseline. Often you should push yourself past the minimum. If you've just got to study for the big exam, you're running a marathon or you're going to spar 10 hard rounds, then you can caffeinate as much as you think you need. But on other days, try to find the minimum.
To know how much is too much of a good thing, you also need to know its opposite -- not enough of a good thing.