Some people only drink coffee in the morning after a long night of partying. Some limit their caffeine intake to an occasional cup of coffee or tea after a terrific meal. And there are those who only indulge if they’re making a special trip to Starbucks for a caramel macchiato, an iced white chocolate mocha or a pumpkin spice latte.

Statistics show, however, that those folks are the exceptions. Nearly two-thirds of Americans drink coffee on a daily basis, and the average adult enjoys (or at least, drinks) more than three cups of coffee a day.

Why do most of us drink so much coffee – or other beverages like tea, caffeinated soft drinks or energy drinks? For some, it may be the taste. For the majority, though, it’s because the effects of caffeine only last a few hours at most. Those who use caffeine to counteract tiredness or to get an energy jolt need to replenish their bodies’ caffeine’s levels regularly, in order to keep the buzz going.

(If it sounds like we’re talking about drug users – we are. Caffeine is actually the world’s most popular psychoactive drug.)

It’s easy to make an educated guess, from experience, at how long caffeine lasts in the body.

But we’re not in the business of educated guesses. We prefer facts. And here they are.

In most cases, it occurs naturally. Caffeine is found in dozens of plants, most of them native to South America, Africa and South Asia. The ones we’re most familiar with, of course, are the coffee plant (which produces coffee beans), Camellia sinensis plants (better known as tea plants), and the cacao plant (which produces the cocoa beans that are turned into chocolate). Other sources of caffeine include kola nuts, guarana berries, guayusa and yerba mate.

(Fun fact: those plants evolved over the years to contain caffeine primarily because it’s a terrific insect repellent.)

Your eye may have caught a mention of kola nuts in that list, and you might have wondered if that’s why cola (and other sodas) contain caffeine. Not exactly. That used to be the reason, but many decades ago, beverage makers stopped using kola nuts to make Coke, Pepsi and similar beverages. They’ve substituted synthetic caffeine instead.

You can probably guess why. Beverage makers claim caffeine is used as a flavor enhancer, but it’s really there because people became acclimated to drinking soda for an energy boost. Synthetic caffeine is used in energy drinks and shots as well. The good news: natural and artificial caffeine are chemically identical and produce the same effects. The rest of the good news: the synthetic version is safe to consume.

Not surprisingly, each beverage and food contains a different amount of caffeine. What you may not realize, though, is that different varieties of caffeinated foods and beverages also have different caffeine levels.

Coffee

Caffeine content depends on a wide variety of factors ranging from the type of coffee beans used and where they’re grown, to grind size, brewing time and temperature.

The average eight-ounce cup of brewed coffee contains 96 milligrams of caffeine, a shot of espresso averages 64mg of caffeine, eight ounces of instant coffee contains about 62mg, and there’s about two milligrams in eight ounces of decaf coffee. (Yes, decaffeinated coffee still contains a little residual caffeine.)

Tea

Even though black tea, green tea and white tea are all made from the same type of tea leaves, green and white tea contain less caffeine, about 28mg. (The exception is matcha, made from powdered tea leaves, which contains 70mg per eight ounces.)

Black tea has almost twice the caffeine because its leaves are oxidized before they’re packaged or used. Herbal teas, with the exception of guayausa and yerba mate, contain no caffeine.

Chocolate

The darker the chocolate, the more caffeine it contains. Dark chocolate has about 12mg of caffeine per ounce, while milk chocolate contains half that amount. White chocolate doesn’t contain caffeine because it’s made from cocoa butter, not cocoa solids.

Soda and Energy Drinks

The amounts of caffeine in soft drinks and energy drinks vary widely because the caffeine isn’t naturally present; it’s added “after the fact.” For example, a 12-ounce can of Coke contains 34mg of caffeine and Pepsi contains 37mg, but Mountain Dew boasts 54mg.

Energy drinks are engineered to contain more caffeine – sometimes, a lot more. For example, Red Bull has 111mg of caffeine in a can, but Rockstar and Monster Energy contain 160mg per can. Looking for a bigger caffeine boost? There are 316mg in Redline Xtreme, and an amazing 350mg in Spike Hardcore Energy. Energy shots brands vary in caffeine content as well, but they average around 200mg.

All of those numbers are interesting, but they’re important as well. The more caffeine you put into your system, the longer its effects on your body will last.

Caffeine is a stimulant that interacts with both the central nervous system (CNS) and the brain’s signaling system. Some have compared its effect on the CNS as being similar to cocaine or amphetamines, because it causes the release of dopamine and serotonin, some of the hormones and neurotransmitters that stimulate positive feelings. Caffeine also stimulates the release of the “fight-or-flight” hormone adrenaline. Even more important, though, is the way that caffeine interacts with adenosine receptors in the brain.

Adenosine is similar to a neurotransmitter, carrying messages throughout the body and brain. To deliver those messages, it binds to adenosine receptors; the receptors are responsible for converting the messages into electrical energy that generate actions in the body.

But here’s what’s important: caffeine can act just like adenosine, binding to the same receptors in the brain – and when they do that, they block adenosine molecules from binding to the receptors and delivering their messages.

The body releases more and more adenosine as the day wears on. Normally, it would tell the body to relax and get more tired, and it would moderate the body’s level of dopamine, serotonin and adrenaline. But since caffeine prevents adenosine from delivering those messages, the body stays awake, alert – and juiced. (You may also have to urinate more often, because caffeine acts as a diuretic when consumed in high amounts.)

Caffeine’s effects don’t last forever, of course. Eventually, adenosine is again able to reach the receptors. That’s why you may feel your heart rate and blood pressure return to normal, and you may begin to get sleepy, as the caffeine buzz wears off. It’s simply adenosine telling your body to relax.

If you don’t put more caffeine into your system, you may experience caffeine withdrawal symptoms about 12-24 hours later (remember, caffeine is a drug). These side effects are usually benign, like irritability and lack of focus, but they can also include serious headaches and even muscle pain.

How long does caffeine stay in the system, and how long does it keep you awake and alert before its effects begin to wear off? Those are both excellent questions.