Why do you not feel sleepy after drinking coffee?

More than likely you have a built immunity to it, when consistently drinking coffee over a period of time you grow immune to the caffeine in it. Same as anything else, you take Tylenol over a period of a few years then you will notice that it doesn't have the same effect as it used to. Let it be known this works for almost anything, Birth Control would work the same way since you ingest it over a long time to stop the hormones, the hormones will adapt and will no longer have the effect it once had, and we know the result of that. The human body is an amazing thing when it comes to adaptation of almost anything.

It could also be learning from Social Psychology studies, that extroverts might feel more prone to sleepiness or tiredness after caffeine (stimulant) as extroverts have already a low cortical level in their system. Meaning they often have to go out looking for stimulation due to their low level.

However since coffee is a stimulant, it satisfies/increases this low level coritcal arousel and hence extroverts will feel tired (satisfied) upon drinking the coffee (stimulant).

Introverts on the other hand may feel more excited or stimulated, as at base level compared with extroverts, they already have a higher level or cortical in their system, hence they do not need to seek stimulation.

Therefore the increase intake of stimulant (caffeine) adds to their already high level of cortical arousel which increases activity or makes them more active.

As people diagnosed with ADD (Attentional Deficit Disorder) are given medication with high levels of stimulant to decrease their need for stimulas sensation seeking, which would calm them down.