Caffeine effects and the reason why you can fall asleep after a cup of black

http://www.chicagoreader.com/chicago/how-does-caffeine-affect-nervous-system-health-research/Content?oid=875717

"The caffeine in coffee works as a stimulant primarily by blocking a neurotransmitter called adenosine. In a normal waking cycle, adenosine gradually accumulates at certain synapses in the brain and in doing so will contribute to increasing drowsiness. But caffeine acts as an adenosine antagonist, blocking the sites to which it binds and preventing it from working — the result is a feeling of increased alertness and energy which begins within a few minutes of consumption and continues for several hours.

However, the body will tend to produce more adenosine to compensate for the fact that adenosine doesn't seem to be working under the influence of caffeine; this means that once the caffeine wears off, increased drowsiness is likely to result. This 'vicious cycle' is responsible for the addictive quality of caffeine, and withdrawal for a heavy caffeine user can be most unpleasant and even medically dangerous.

I am aware of no clear-cut pharmacological reason that caffeine in and of itself should make a person feel sleepy rather than wakeful, but I can think of several murky possibilities:

• The coffee you are drinking may not be very "strong," and is perhaps weakly brewed with a low caffeine content. Try ordering an Americano with two shots of espresso and drinking it quickly on an empty stomach; if that makes you sleepy, I'd be concerned about your planet of origin! 
• Your coffee may be heavily sweetened, and the drowsying effects of the associated sugar crash may be stronger than the opposite effects of the caffeine.
• There is some evidence that stimulants affect persons with ADD/ADHD much differently than others, though this distinction is mainly applied to a family of stimulant drugs which are dissimilar to caffeine, and your reaction to caffeine shouldnot be taken as a marker for such disorders.

Caffeine does seem to affect different people in different ways — but caffeine does not chemically induce drowsiness as a primary effect and would never be used in such a way by a physician, for example, so it is probable that something quite funny is going on if you find that even large doses of caffeine routinely make you sleepy."

"It depends on the time course.  I’d be surprised if somebody would 

drink a cup of coffee then immediately feels sleepy.  But I think it’s perfectly reasonable to feel sleepy a little bit later on because what's happening when you drink the coffee is that caffeine is complicated but its main effect is to block the action of a transmitter called adenosine which is a sort of dampening down a transmitter if you like in the brain.  So, when you block the effect then you feel more awake.  Later on, when the coffee effect wears off after it gets eliminated in every 4 hours or so, so when that's gone, the adenosine is still chugging along there and then that may then switch the other way so you feel more sleepy.  So, I don’t know of a circumstances where you would drink coffee and then suddenly feel sleepy."

http://science.howstuffworks.com/caffeine4.htm