I was just thinking about this (and having one) when I came to the conclusion, I don’t know what causes them. I know that it has something to do with withdrawal.
What are the physiological reasons behind a caffiene headache, brought about from lack of caffiene?
It’s the caffeine. Caffeine is an adenosine receptor site blocker, just like cocaine is a dopamine receptor site blocker. In an effort to maintain homeostasis your body will reduce the number of receptor sites near the synapses. When you lose the stimulus of caffeine you have already “fatigued” those receptor sites and they don’t potentiate as often. Withdrawl.
What’s really fun is when you have jetlag. Wake up at 3:00 am with a splitting caffiene headache. Of course, the last thing you want is caffiene 
Wait - Kid - you mean the number of receptor sites can change from moment to moment? How so? This is totally new to me.
One of the television news magazine shows (Dateline or 20/20 or some such) did a story a year or so ago about how many OTC headache relief medicines (like Excedrin) contain caffeine. So people who try to kick the coffee habit and then wind up taking these headache pills because of the withdrawal symptoms wind up still feeding their caffeine addiction. And you’ll still have to go through withdrawal headaches when you stop taking the pills.
I don’t drink coffee, but I can down Dr. Pepper by the gallon–err, liter-- and I have gone cold turkey off caffeine many times before. The headaches were not fun, but they only lasted a day or two and I find that I feel much better physically and mentally when I am not hooked.
CC -
First off IANAD or a BC but from what I know -
I wouldn’t say “moment to moment.” We’re talking chronic use here. Your body will take a number of steps to maintain homestasis some of which include decreasing endogenous neurotransmitters as well as the number of sites for them to bind.
I believe also that the caffeine dialates the blood vessels in the head, and when you go off of it, the vessels scrunch back down and yo’ head doesn’t get the blood it is used to.
KidCharlemagne, cocaine doesn’t block dopamine receptors. It does inhibit dopamine re-uptake, which reults in an increase in the amount of dopamine in the synapse.
Sorry for the brief hijack.
Joe Mahma,
Yea my bad, cocaine attaches to the dopamine transporter making it impossible to be reabsorbed. Still a block but not of the same type.