Around 90 percent of Americans consume caffeine in one form or another every single day. More than half of all American adults take more than 300 milligrams (mg) of caffeine every day, making it America's most popular drug by far.

Have you ever wondered what it is that makes caffeine so popular? What does this drug do that causes its use to be so widespread?

Caffeine is known chemically as trimethylxanthine. Medically, caffeine is a cardiac stimulant and also a mild diuretic (it increases urine production). Recreationally, caffeine provides a "boost of energy" or a feeling of heightened alertness. Many people feel as though they cannot function in the morning without the boost that caffeine gives them.

Caffeine occurs naturally in many plants, including coffee beans, tea leaves and cocoa nuts. So you find caffeine in lots of different food products. Let's look at the most common sources of caffeine for Americans.

Typical drip-brewed coffee contains 100 mg per 6-ounce cup. But who drinks a 6-ounce cup of coffee anymore? If you are buying your coffee at a coffee shop or convenience store, or drinking it at home out of a big mug or a commuter's cup, you are consuming it in 12-, 14- or 20-ounce containers. You might be getting 300 mg of caffeine with every "cup" you drink. Typical brewed tea contains 70 mg per 6-ounce cup.

Typical colas (Coke, Pepsi, Mountain Dew, etc.) contain 50 mg per 12-ounce can. Things like Jolt contain 70 mg per 12-ounce can. Red Bull has 80 mg in its little can. You may have heard the news that an energy drink called "Spike Shooter" was banned in Colorado. The problem: when some high school students drank the stuff, they had to be hospitalized because of a caffeine overdose. Each can contains 300 mg of caffeine.

Even pills can contain caffeine. Anacin contains 32 mg of caffeine per tablet. Some weight-loss pills contain as much as 200 mg per tablet.

Now you can see why half of all American adults consume more than 300 mg of caffeine per day. Two big cups of coffee are all you need to get you there. Many people consume a gram or more every single day and don't even realize it.

Why does caffeine wake you up?

When you get sleepy, it is because of a chemical called adenosine. Adenosine builds up in the brain whenever you are awake. Adenosine binds to adenosine receptors on brain cells. The binding of adenosine causes drowsiness by slowing down nerve cell activity.

To a nerve cell, caffeine looks just like adenosine. So caffeine binds to the adenosine receptors in your brain. However, caffeine doesn't slow down the cell's activity like adenosine would. So the cell cannot "see" adenosine anymore because caffeine is taking up all the receptors adenosine binds to. Instead of slowing down because of the adenosine level, the cells speed up.

So now you have lots of neurons firing in the brain. The pituitary gland sees all of the brain activity and thinks some sort of emergency must be occurring, so it releases hormones that tell the adrenal glands to produce adrenaline. Adrenaline is the "fight or flight" hormone. Your pupils dilate. Your heart beats faster. The liver releases sugar into the bloodstream for extra energy. This explains why, after consuming a big cup of coffee, your muscles tense up, you feel excited and you can feel your heart beat increasing.

Caffeine also increases dopamine levels in the same way that amphetamines do. Dopamine is a neurotransmitter that, in certain parts of the brain, activates the pleasure center. It is suspected that the dopamine connection contributes to caffeine addiction.

So you can see why your body might like caffeine in the short term, especially if you are low on sleep and need to remain active. Caffeine blocks adenosine so you feel alert. It injects adrenaline into the system to give you a boost. And it manipulates dopamine production to make you feel good.

The problem with caffeine is the longer-term effects, which tend to spiral. For example, once the adrenaline wears off, you face fatigue and depression. So what are you going to do? You take more caffeine to get the adrenaline going again. As you might imagine, having your body in a state of emergency all day long isn't very healthy.

The most important long-term problem is the effect that caffeine has on sleep. Adenosine is important to sleep, and especially to deep sleep. After drinking caffeine all day, your body probably will miss out on the benefits of deep sleep. The next day you feel worse, so you need caffeine as soon as you get out of bed. The cycle continues day after day.

As you might suspect, modern technology is busy creating the "next big thing" to replace caffeine. These drugs are called eugeroics. They keep you alert without all the side effects of caffeine. Provigil is probably the best-known eugeroic right now, but more are on the way. One day soon, people may look at caffeine in the same way that we look at the horse and buggy.