As adenosine is created in the brain, it binds to adenosine receptors. This binding causes drowsiness by slowing down nerve cell activity. In the brain, this also causes blood vessels to dilate to let more oxygen into that organ during sleep. To a nerve cell, caffeine looks like adenosine: Caffeine binds to the adenosine receptor. However, caffeine doesn’t slow down the cell’s activity like adenosine would. As a result, the cell can no longer identify adenosine because caffeine is taking up all the receptors that adenosine would normally bind to.
This video from How Stuff Works shows how caffeine tricks nerve cells into believing it is adenosine:
Caffeine also causes the brain’s blood vessels to constrict, because it blocks adenosine’s ability to open them up. Brain cells grow more adenosine receptors, which is the brain’s attempt to maintain equilibrium in the face of a constant onslaught of caffeine, with its adenosine receptors so regularly plugged. This explains why regular coffee drinkers build up a tolerance over time—because you have more adenosine receptors, it takes more caffeine to block a significant proportion of them and achieve the desired effect.
If you have been taking caffeine every day and you stop, you can get an incredible headache because of the increased blood flow in your brain. The adenosine receptors are able to function properly again. Your brain is used to operating in one set of conditions (with an artificially-inflated number of adenosine receptors, and a decreased number of norepinephrine receptors) that depend upon regular ingestion of caffeine. Suddenly, without the drug, the altered brain chemistry causes all sorts of problems, including the dreaded caffeine withdrawal headache.
You only need to get through about 7-12 days of symptoms without drinking any caffeine. During that period, your brain will naturally decrease the number of adenosine receptors on each cell, responding to the sudden lack of caffeine ingestion. If you can make it that long without a cup of coffee or tea, the levels of adenosine receptors in your brain reset to their baseline levels, and your addiction will be broken.
The frontal cortex of our brain has been known to affect our behaviour. It controls the capacity to plan, reason, conduct higher-level thinking and connect what we know about the world to how we behave. Blood gives this essential region of the brain oxygen, nutrition, and it takes away waste from the cells located there. Is it wise to decrease the blood flow to the area of our brains which helps us plan, reason, and conduct higher level-thinking?