One thing most people can agree on is that caffeine works! In fact, 85% of Americans reach for caffeine as part of their daily ritual. But how exactly does caffeine get the job done? How does the body process and get rid of it? And why can one person enjoy an espresso after dinner and sleep like a baby, while others steer clear of caffeine after lunch time? To help lay out the facts, we’ve compiled the top seven things you should know about how caffeine works in the body.

1. The absorption of caffeine in the body can take 15­‑45 minutes. This is in part due to the fact that caffeine is amphiphilic, a fancy way to say that it can move freely through all membranes of the body. This feature gives caffeine the ability to pass quickly through the blood-brain barrier, where it goes to work fast. While it might be tempting to reach for a second cup of joe right after finishing your first in the morning, next time you may try waiting an hour until the caffeine you’ve had is fully absorbed.
2. Caffeine works because it’s a doppelganger. The shape of the caffeine molecule is similar to that of adenosine, a molecule that calms brain activity, dilates blood vessels and promotes relaxation to help us fall asleep. Because caffeine is shaped like adenosine, the two compete to bind to receptors in the brain. When caffeine wins, it not only prevents adenosine from doing its job, but also makes your nerve cells fire faster and your blood vessels constrict, typically causing feelings of wakefulness and alertness.
3. Drinking caffeine quickly doesn’t make it work faster. While it might seem like faster consumption will lead to faster absorption, the speed at which a caffeinated beverage is consumed and the temperature of the beverage don’t appear to meaningfully change the rate of absorption in the body. When managing caffeine intake, dosage is the most important factor to monitor. Check out our helpful caffeine guide to learn more about common types of caffeine and their amounts.
4. The typical half-life of caffeine ranges from 5-7 hours, though wider ranges have been reported (2.3-9.9 hours). A half-life in the context of nutrition is the amount of time it takes for the body to eliminate half of the original dose of a molecule. The wide range in half-life for caffeine exists due to a number of factors, including caffeine-consumption habits, age, genetics, overall health, and lifestyle choices. Understanding half-life can help you estimate and manage the best time to have your last coffee, cola or energy drink of the day. For example, if you enjoy a cup of coffee with about 100 milligrams caffeine around 3:00 pm, by 9:00 pm, when you’re ready for bed, about 50 milligrams of that caffeine may still be present. In other words, what may help get you through the afternoon may also have a negative impact on your sleep quality that night.
5. Leave the heavy lifting to the liver. After perking you up, caffeine is ultimately broken down (metabolized) by the liver into three major metabolites: paraxanthine (84%), theobromine (12%), and theophylline (4%); all three of which are also active ingredients. After the liver metabolizes caffeine, the kidneys excrete what’s remaining into urine.
6. Lifestyle choices can impact how caffeine works. A myriad of factors exists that can alter the way caffeine is processed in the body, which is why a one-size-fits-all approach to dietary caffeine advice is often challenging. Beyond age and genetics, lifestyle choices such as smoking and diet can influence how caffeine is metabolized. For instance, those who smoke metabolize caffeine twice as fast as non-smokers. It was also previously thought that certain oral contraceptives slow down caffeine metabolism and that the phase of one’s menstrual cycle may affect the rate at which caffeine is metabolized. The key is that everyone likely has a slightly different experience with caffeine.
7. Pregnancy is a different ball game. During pregnancy, caffeine’s half-life increases (to 11.5-18 hours), especially toward the third trimester. Caffeine stays in the pregnant body longer because it crosses the placental barrier, where neither the fetus nor the placenta has the enzymes to metabolize it. Health practitioners generally advise women who are pregnant to reduce or eliminate caffeine from their diet, though major health organizations suggest that up to 200 milligrams per day is still safe during pregnancy.

The way caffeine fits into your lifestyle, if at all, is both learned and personal. When seeking specific advice, it’s best to consult with a health practitioner. In general, be patient after consuming caffeine to allow enough time for your body to absorb it. Similarly, be sure to factor in enough time before your target bedtime to let your body process your favorite coffee, tea, cola or energy drink. However you choose to enjoy caffeine, know your best dosage by learning where it can be found on food labels and how much your favorite products contain.