The FDA reports that more than 80 percent of American adults drink coffee daily. Regular ingestion of caffeine alters your brain’s chemical makeup, leading to fatigue, headaches and nausea if you quit.
It all happens within 24 hours. It's not bad at first, but you begin to feel mentally foggy and lack alertness. Then fatigue sets in even if you haven’t done anything strenuous and you you begin to get irritable.
Caffeine is chemically addictive and now the Diagnostic and Statistical Manual of Mental Disorders has included caffeine withdrawal as a mental disorder for the first time.
Soon after you consume caffeine it’s absorbed through the small intestine and because it's water and fat soluble it penetrates the brain. Structurally, caffeine closely resembles a molecule that’s naturally present in our brain, so much so that caffeine actually fits into our brain cells’ receptors for adenosine. Adenosine produced over time locks into these receptors and produces a feeling of tiredness, but caffeine blocks off the adenosine. The result is that caffeine produces a sense of alertness and energy for a few hours. Also, a natural stimulant, dopamine, works more effectively when the receptors are blocked. In addition, surplus adenosine causes the adrenal glands to secrete adrenaline.
Technically, caffeine isn’t a stimulant, but a substance that stimulates the body's natural stimulants. Caffeine's effects last for anywhere from four to six hours, depending on the person’s age, size and other factors.
People who ingest caffeine on a regular basis experience changes in the brain’s chemistry and physical characteristics over time. For one, the brain cells grow more adenosine receptors in an attempt to maintain neutralize the constant doses of caffeine. This explains why coffee drinkers build up a tolerance and it takes more caffeine to achieve the desired effect.
It also explains why suddenly giving up caffeine entirely triggers withdrawal effects. Your brain is used to functioning with an artificially inflated number of adenosine receptors that depend on regular doses of caffeine. Without it the altered brain chemistry causes all sorts of physical symptoms.
To get off caffeine you to suffer through about 7-12 days of symptoms without ingesting any caffeine. During that period, your brain decreases the number of adenosine receptors and they will return to normal. Further reading...What Caffeine Actually Does To Your Brain