Along with its ability to ward off drowsiness, caffeine also increases dopamine levels the same way amphetamines do. Dopamine, a neurotransmitter, activates pleasure centers in certain parts of the brain. Cocaine and heroin also manipulate dopamine levels. They do this by slowing the rate of dopamine reabsorption. Caffeine's effect, obviously, is much lower than cocaine's, but it is the same mechanism. Scientists suspect the dopamine connection contributes to caffeine addiction.
It's obvious why your body likes caffeine in the short term, this is especially true if you're low on sleep and need to remain active. It blocks adenosine reception, making you feel alert. It stimulates adrenaline production to give you a boost. It also manipulates dopamine production to make you feel good.
Unfortunately, caffeine's longer-term effects tend to spiral. Once the adrenaline wears off, for example, you face fatigue and depression. To combat this, you consume more caffeine and get that adrenaline going again. But remaining in a state of emergency all day isn't very healthy. It also makes you jumpy and irritable.
Caffeine's most important long-term problem is the effect on sleep. The half-life of caffeine is about six hours. If you gulp down a big cup of coffee (with its 200 mg of caffeine) at 3 p.m., by 9 p.m. about 100 mg is still in your system. If you can fall asleep, your body probably will miss out on the benefits of deep sleep. The lack of deep sleep adds up fast. The next day, when you feel worse, you crave caffeine as soon as you get out of bed. Day after day, that cycle continues.
Now, 90 percent of Americans consume caffeine every day. They get in the cycle, and have to keep consuming the drug. What's worse: if they stop consuming caffeine, they get very tired and depressed, and get a terrible, splitting headache from the blood vessels in the brain dilating. These negative effects can force you to run for more coffee even if you want to stop.