Remember the body producing all those "extra" adenosine molecules? Well they aren't just flushed out if they aren't used. They hang out and wait out the caffeine molecules deterioration which typically takes 5-6 hours (hence 5 hour energy drinks name). When the caffeine molecules deteriorate they fall out and enzymes break them up and flush them out of your body through the blood stream.
As they start to deteriorate and fall out, the adenosine molecules are quick to plug into the hole so other caffeine molecules don't plug in (but mainly because it's their nature- their life's mission). At some point there are going to be a lot of these falling out and adenosine plugging in, but since there are SO MANY extra adenosine molecules to plug in, they start to overstimulate your CNS (jitters).
Since the caffeine wasn't an active molecule the receptor, which is basically a nerve ending, becomes used to not being stimulated. Imagine if you were to slowly tap on your arm then progressively tapping harder and harder- you'd build a tolerance for the pain and your nerves would adjust. If, however, you were to start out with a hard tap, this would hurt and likely leave a lasting painful "bruise" of a sort. If you kept going without building tolerance this would really start to hurt.
This is exactly what's happening. The adenosine molecules are plugging in and activating/stimulating that "nerve" without time to build up tolerance and since the body is in def con 1 at the time the adenosine needs to hit the breaks and crash you for your own good- to stop you from damaging yourself further!
This is the crash AND the headache you get from caffeine. The irony is the caffeine just antagonized the effect- it's actually the adenosine that's causing the headache- and it was built by your body and nutrition- not caffeine. Sounds a bit like an oxymoron effect doesn't it?
I'm not done. There is collateral damage as well.
As if this all wasn't enough, there is the issue with all the carnage you left in your energetic wake. All the while your pushing yourself beyond your own limitations (based on available nutrition, hydration etc...) and the lack of "supervision" of your neurotransmitters, you burned them all out. Many, however couldn't plug in fast enough and were destroyed. The body, realizing that producing them was only going to lead to depletion faster than the body could create more, goes into hibernation mode and slows to a crawl the production also to protect you from yourself.
This means available dopamine, serotonin, and endorphins are being recalled and locked up. If this happens too much as is the case with power slamming energy shots, drinks, candy all day and every waking moment to get more energy.
The new coffee, for example after your energy effect has diminished, but you haven't yet depleted all your chemicals- just forced them into lock-down.
This also has the side effect of the body not producing more, it may need those "building blocks" your food gave it to keep you alive so no more serotonin, dopamine, and so on. So your left with ONLY what was already created before, and if you keep going like you are, you will use up your entire supply.
Drinking that next energy drink, coffee, etc... has the effect of forcing out those neurotransmitters, those feel good chemicals like a jail break- and you end up using them up thus coffee stops working because you have no resources to use to create energy, heightened feelings of well being, and even your adrenalin gets used all up.
That's when you really crash hard- you basically used up your entire supply for the day in a few hours and are now useless- and drinking more coffee won't do anything any more than trying to gamble at the casino after blowing all your money and having no more in the bank will allow you to continue to play.
Now your depleted (and likely starving), your head is killing you, and you feel like absolute dung.
The second fold I mentioned above is you can't function due to lack of proper regulation transmitters (neuro-transmitters), so your double kicked in the groin by your body for bringing your jerk friends over to annoy your "roommates" cells and for raiding your fridge (neurotransmitters/nutrients), getting high (blocking the adenosine receptors blocks the body from hitting the breaks thus not maintaining that "energy" you feel when you drink coffee... and those jerks left a mess behind (you have to pick up the pieces after a caffeine rush).
The addiction from caffeine starts out as a mental problem. You rationalize that your tired, really tired, so you need more caffeine. You drink more and the energy returns, but this is at the same expense but worse- as the first round.
The mental addiction is of the "feeling of power and motivation" you felt- you got high on your body's chemicals and used them up- now you have nothing to "carry you" through the day and now must rest, eat nutritious foods to replenish the building blocks and return to normal. Was it worth it?
Meanwhile your neurotransmitters (call them your energy for this illustration) already being down regulated (so eating more nutrients replenishes them but the body refuses to create more due to the "law" passed by the congress of your receptors and does nothing but store them or you metabolize them out for life support functions like digestion).
The more you drink the deeper the down regulation thus the more drastic your weak, tired feeling.
Now what happens next is your on your 4th cup of coffee- and it's not working anymore. This is because the effects make use of the neurotransmitters themselves (directly and indirectly) and since each cup is down regulating the production, you have less and less "fuel" to fire up the body/brain.
Now your burning out your neurotransmitters like working at walmart on black friday with 2 employees in the electronics department when 10 would be required to handle the demand put on the department!
see where this is going?
Now this is just a day- take this on for several months- you start to realize coffee sucks, but worse you don't even realize it's pushing you into stupid-hood, making life harder to manage, and is the very culprit of many problems you blame elsewhere (overworked at your job, when really your not, your just finding it harder to cope and thus even a regular load of work seems impossible- now your calling your boss a slave laborer when it's in reality your bad habits that are deteriorating your ability to cope with just what's already expected of you- grumpiness ensues - and your blaming everything but the real gangster molecules responsible- caffeine.
Once your neurotransmitters are taxed to death you crash. Rebound headache makes things worse when you stop drinking coffee.
Since the caffeine molecules jacked the receptors AND DIDN'T activate them this is like a thousand people entering your job at the same time demanding your attention all at once and your too tired to deal with it. Patience is gone because patience is a result of efficiency, having enough feel good chemicals to counter stress!
One effect of the over production of adenosine molecules earlier is the brain tries to cope by creating or rather activating, more receptors in a desperate attempt to hit the breaks. Your body is a run away train and your doing it to yourself because you want that rush!
Your like a kid blowing his entire allowance on candy, then you eat it all at once and get sick and wonder what happened. Now when you crash you have an abundance of adenosine receptors and adenosine to fit into them and you get mob rushed.
Since adenosine molecules/receptors are actually designed to relax you and make you tired to stop you from overexertion, and you have more than your supposed to like feeding a Gremlin after midnight, this leads to extreme fatigue, like all the boards were removed and all the breaks on the train went down at once- as well as a mad headache from those under-stimulated receptors all being stimulated at once (the whole head headache and back ache is the result of this pounding). If you avoid caffeine, drink lots of water and wait it out, as painful as it is, the adenosine receptors will eventually down regulate (the CNS will turn them off and lock the doors so adenosine cannot plug in or bother you in those receptors- then you slowly reach homeostasis (balance) and feel better. The best way to relieve the pain is to relax, try not to think, move, or open your eyes if possible.
It's not a pain headache so much as an effect similar to spending all day in the sun, getting a horrible sunburn then someone constantly slapping you on the back, giving you bear hugs- essentially stimulating those nerve endings- but in your brain.
Doesn't this make you want to sip that coffee next to you?