I f you're like most American adults, who roll out of bed every morning and make a blurry-­eyed shuffle across the house to the coffee pot, you're probably so happy that caffeine does work, you've actually never cared to know how it works. But wake up and smell the science. That morning Macchiato might seem like it can do magic, but it's actually causing a chemical reaction in your brain that's more complicated than brewing up a good cup of chai.

Average Joe

Forget cigarettes and alcohol, caffeine is America ’s most popular drug by far. More than half of all American adults consume 300 milligrams of caffeine or more every day. That might sound like a lot, but two mugs of coffee or a mug of coffee and a couple of sodas during the day are all you need to get there.

Medically, caffeine is useful as a cardiac stimulant and also as a mild diuretic, because it increases urine production. Recreationally, however, it's used to provide people with a boost of energy or a feeling of heightened alertness. Caffeine achieves this effect by triggering the same mechanisms that amphetamines, cocaine and heroin use to stimulate the brain, albeit to a lesser extent. But it does manipulate the same brain channels, which is what gives caffeine its addictive qualities and qualifies it as a drug.

The Best Part of Wakin' Up ...

Caffeine works in three different ways to produce feelings of alertness inside the body. The first is by blocking the amount of adenosine in the brain. Adenosine is a chemical created in the brain that, when it binds to adenosine receptors, causes drowsiness by slowing down nerve cell activity. Adenosine binding also causes blood vessels to dilate, presumably to let more oxygen in during sleep.

When caffeine enters the body, however, it prevents adenosine binding. To a nerve cell, caffeine "looks" like adenosine, so it's able to bind to adenosine receptors. Of course, caffeine doesn't slow down the cells' activity like adenosine would. Instead, the cells speed up because they can't "see' adenosine, since the caffeine is occupying all the adenosine receptors. Similarly, since the brain's blood vessels dilate with adenosine, caffeine causes them to constrict. This is why some headache medicines like Anacin contain caffeine; if you have a vascular headache, the caffeine will close down the blood vessels and relieve it.

All of this restricted adenosine activity in the brain sets into motion caffeine’s second major effect on the body. Now that you have increased neuron firing in the brain due to lack of adenosine, the pituitary gland thinks some sort of emergency must be occurring. Consequently, it releases hormones that tell the adrenal glands to produce adrenaline (epinephrine), the "fight or flight" hormone. The adrenaline then triggers a number of different physical changes in the body: your pupils dilate, your heart beats faster, blood flow to the stomach slows, your liver releases more sugar into the bloodstream for extra energy, and your muscles tighten. So it's no wonder why, after consuming a big cup of coffee, your hands get cold, your muscles tense up, and you can feel your heartbeat increasing.

Finally, caffeine is able to activate the body's level of dopamine, a neurotransmitter that, in certain parts of the brain, activates the "pleasure center:' This makes all that jitteriness not so bad after all. When caffeine enters the brain, it increases dopamine levels just like heroin and cocaine by slowing down the rate of dopamine re absorption into brain neurons. Obviously, caffeine’s effect is much less than heroin's, but it is the same mechanism, which is why it's suspected that the dopamine connection contributes to caffeine addiction.

Keeping You Coming Back for More

It's obvious now why your body likes caffeine so much when you're low on sleep and need to remain active. That innocent looking cup of coffee or soda blocks your brain's adenosine reception so you feel alert, then injects adrenaline into your system to give you a boost. And on top of all that, it manipulates dopamine production to make you feel good.

But nothing comes without a cost. Caffeine’s short-term effects are pleasant, but its long-term effects tend to spiral. Once the adrenaline wears off, you face fatigue and depression, which is what makes most of us reach for that second cup of joe. But, ultimately, this is equivalent to keeping your body in a state of emergency all day long, which, as you can imagine, isn't very healthy, and can make you jumpy and irritable.

The most important long-term problem, however, is the effect that caffeine has on sleep. It inhibits adenosine reception, which is important to sleep, especially to deep sleep. The half­ life of caffeine in your body is about six hours. That means that if you consume a big cup of coffee with 200 mg of caffeine at 3 p.m. , by 9 p.m. about 100 mg of that caffeine is still in your system. You may be able to fall asleep, but your body probably will miss out on the benefits of deep sleep. That deficit adds up fast. The next day you're likely to feel groggy, so you crave caffeine as soon as you get out of bed. Even worse, if you try to stop taking caffeine, you get very tired and, and you get a terrible, splitting headache as blood vessels in the brain dilate. The cycle continues, which probably explains why 90 percent of Americans consume caffeine every day.