Caffeine is undeniably our drug of choice—but what exactly does it do to our bodies?

“Sorry, I haven’t had my coffee yet.” It’s a blanket excuse for forgetfulness, bad moods, late arrivals and more. When a naturally occurring substance can promise reduced fatigue, increased arousal, improved concentration, and mood enhancement like caffeine does, it’s no wonder we treat it like magic and use it in droves. According to the FDA,  80% of adults consumer caffeine every day, and though our coffee drinking has actually decreased by nearly half in since the 1940s, there are more caffeine delivery methods (think energy drinks, sodas, gels, capsules) than ever. 

So what does this substance actually do in our bodies? “The primary way caffeine functions is that it blocks a neurotransmitter in our brains called adenosine, which tells the brain when we’re tired,” explains nutritionist Elisa Zied, MS, RD, CDN, author of Younger Next Week:  Your Ultimate RX. “Caffeine looks enough like adenosine that it’s able to sit in place of that neurotransmitter so the brain never gets the ‘I’m sleepy’ message,” adds Murray Carpenter, author of Caffeinated:  How Our Daily Habit Helps, Hurts and Hooks Us. And this imitation is a large-scale production. According to Carpenter, when you’re caffeinated, 50% of your adenosine regulators are being blocked. 

Caffeine doesn’t just ward off fatigue—it’s also a powerful physical enhancer that allows for increased concentration. In the brain, caffeine creates increased neuron firing, which causes the pituitary gland to release hormones that signal the adrenal glands to secrete adrenaline. That’s why after a few cups of java, you may spot some of these signs: dilated pupils, increased heart rate, rising blood pressure and tightened muscles. Is your energy up? You bet. 

“Say you’re in a marathon and you lose focus 20 miles in,” says Carpenter. “Caffeine can help you maintain that focus and also help your muscles to contract slightly more forcefully.” Carpenter calls it the “double whammy” of increasing both mental and physical energies and he notes that even a moderate dose, like two cups of coffee, will improve athletic performance for most people—not just athletes—by 1-3%. 

Another affect: “Caffeine makes you feel good,” says Carpenter. That’s because a second neurotransmitter that’s affected by caffeine, dopamine, activates the brain’s pleasure centers. Caffeine can slow the reabsorption of dopamine in a way that’s similar to what heroin and cocaine do (on a much lesser scale). Also, it’s legal, so that’s a bonus.

Caffeine’s not a perfect substance, though. It can have negative affects on our health, especially when it’s delivered through sugary drinks that are more like desserts than beverages and pack major calories (hi, Mocha Frappuccinos!). Sodas and energy drinks—which are often marketed to children and adolescents—drive cravings for sweet products and contribute to obesity, and all of these beverage varieties deliver more caffeine than you may realize. “Decadent coffee beverages can pack in tons of caffeine, more than the 300-400 milligrams recommended as a maximum daily amount,” says Zied. “Their size is a problem, and I also recommend drinking coffee or tea with as little added sugar as possible.” With these drinks, excess sugar is a given.

And then there’s the sleep thing. The Center for Disease Control and Prevention has called insufficient sleep “a public health epidemic,” and studies clearly show that consuming caffeine, especially in the afternoon hours, can negatively impact sleep habits. It all goes back to the adenosine imitation that caffeine does so well, blocking our body’s ability to recognize fatigue. “So many Americans sleep poorly, and so many Americans drink caffeine,” says Carpenter. “It’s easy to get into a vicious cycle of treating sleep deprivation with excessive caffeine use, but let’s be clear: You will sleep better without caffeine in your diet.” Zied agrees, but adds that sticking to a cup or two of coffee (8-16 ounces total) in the morning is doable for people who don’t have adverse responses to caffeine.

The Bottom Line:

Caffeine has both positive and negative affects on our bodies, and it’s hard to sort through the ever-increasing research and information being published. Zied notes that no one study should dictate whether you include caffeine in your routine. “It's important to look at your personal story, your lifestyle, and how caffeine may fit, or not, into your otherwise healthful and balanced diet,” she says. Still, if you partake in a daily caffeine habit, it’s key to know what it does to your body so that you can consider an amount, timing and rate of consumption that works for your own life. “This is a powerful, ubiquitous drug that most Americans take daily in some form,” Carpenter says. “It deserves a lot more respect than it gets.” 


  1. FDA: Caffeine and Your Body
  2. Facebook: Caffeinated the Book
  3. Harvard School of Public Health
  4. CDC: Insufficient Sleep
  5. Elisa Zied