In his book Me, Myself, and Us: The Science of Personality and the Art of Well-Being, psychologist Brian Little argues that introverts shouldn't drink coffee before an important meeting, or anything like it.
Why does coffee seem to have this effect on introverts?
This isn't my own research, but it's based on the theory of extraversion by Hans Eysenck and research by William Revelle of Northwestern University. It's the idea that introverts and extraverts differ in the level of neocortical arousal in the brain — in other words, how alert or responsive you are to your environment. According to this theory, introverts are over the optimal level — that is, more easily stimulated — and extraverts under the optimal level.
It's more complex than that, but this is a useful model because it allows us to make some predictions. This suggests that performance will be compromised for introverts if they are exposed to stimulating situations, or if they ingest a stimulant (such as caffeine),which pushes them even further away from the optimal level.
So when should introverts have their coffee, then?
Later in the day would be better; at any rate, they should try not to have caffeine right before something like an important meeting, as I say in the book.