841. There’s definitely a lot of individual variation. But knowing the half-life of caffeine is useful…especially if you actually understand what the half-life of a substance actually means.
There’s definitely a lot of individual variation. But knowing the half-life of caffeine is useful…especially if you actually understand what the half-life of a substance actually means.
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#620: Is coffee good or bad for you?
#361: How to find coffee that won’t bother your stomach
Find a transcript here.
How late is too late in the day to drink caffeine? There’s definitely a lot of individual variation here. But knowing the half-life of caffeine is useful…expecially if you actually understand what the half-life of a substance actually means!
Hello! I’m Monica Reinagel and this is the Nutrition Diva podcast, a show where we take a closer look at nutrition news, research, and trends and answer your food and nutrition questions.
A listener recently wrote to me after reading that caffeine has a half-life of six hours, wondering what that actually means.
This is such a good question because while most of us are familiar with the term “half-life,” few of us have a good grasp on what it actually means. You usually hear it applied in the context of medications and not foods or nutrients. But even though it’s a natural compound, caffeine is actually a drug. And that means that the tools that we use to measure how drugs move through the body apply here, as well. So let’s get into this.
When scientists talk about the half-life of a substance, they’re describing how long it takes for the level of that substance in your bloodstream to drop by half. Now, that clock doesn’t start the moment you swallow a pill—or, in the case of caffeine, take a sip of coffee. There’s a short delay while your body absorbs it. But once it’s fully absorbed and circulating, half-life refers to the speed at which your liver and kidneys will clear it from your system.
So, if something has a two-hour half-life, that means that once that substance has peaked in your bloodstream, it will take about two hours for your body to reduce that concentration by half.
At this point, you might be wondering why we don’t just measure the total time it takes to leave the body altogether. That would seem simpler, right? The reason we don’t is that this decline is not linear.
If the half-life of a substance is two hours, it sounds like it should take four hours to eliminate it completely. But, in fact, it’ll take 8-10 hours before it’s completely out of your system. And this is because the clearance rate slows down as the blood concentration of the substance gets lower.
So, in the first half-life, it goes from 100% to 50%. In the second half-life, it goes from 50% to 25%. In the third interval, it goes from 25% to 12%, and so on. This is referred to as “first order kinetics,” and most drugs follow this pattern.
There are a couple of exceptions to this, the most notable of which is alcohol. If you have more than 1 (or at most two) standard drinks, zero-order kinetics take over, meaning that no matter how much alcohol is in your system, your body will only be able to clear about 7-10 grams of alcohol per hour–which is the equivalent of ½ to ¾ of a standard drink.
But, as I said, for most substances, the rate of clearance depends on how much is present. When there’s a lot circulating, your body clears more per hour. As levels drop, the process slows down.
Half-life isn’t only about figuring out when something will be gone from your system. Doctors and pharmacists also use half-life to design dosing schedules that maintain a consistent concentration of a medication in your bloodstream. The same math also tells them how long it will take to reach what’s called a steady state, where the amount coming in with each dose equals the amount being cleared out.
It’s also important to remember that a half-life isn’t a fixed, universal number. It’s an average, based on studies of large groups of people. In real life, there’s a lot of individual variation: your age, how well your liver and kidneys are functioning, your body composition, genetics, pregnancy, other medications you may be taking, and even whether or not you smoke can all change how quickly your body metabolizes and clears a substance. Some people are fast metabolizers; others are slow.
OK, now that we have a working understanding of half-life, let’s get back to the question at hand: How late in the day can you really afford to have that second (or third) latte?
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Caffeine is the world’s most widely used psychoactive drug. What most of us enjoy about caffeine is that it’s a central nervous system stimulant. Specifically, it blocks the neurotransmitter adenosine, which builds up over the day and makes us feel sleepy. By temporarily putting adenosine on mute, caffeine sharpens alertness, boosts concentration, and makes us feel more energetic and upbeat.
Of course, that same mechanism also explains caffeine’s less charming side effects. When adenosine can’t do its job, the nervous system stays in a more “switched-on” state. For some people, that leads to jitteriness, irritability, or difficulty falling asleep.
And here’s where half-life comes back into play. On average, caffeine’s half-life in healthy adults is around six hours. Let’s say your morning coffee contains about 200 milligrams of caffeine. Six hours later, at lunchtime, you still have around 100 milligrams circulating. By dinnertime, you’re down to about 50 milligrams—roughly the caffeine in a small cup of tea.
Now remember, that six-hour half-life is only an average. For some, it might be as short as three or four hours; for others, it can stretch to ten or even twelve hours.
Either way, however, if you have another coffee mid-afternoon, you’re not starting from zero. You’re layering new caffeine on top of what’s still decaying from your morning dose. That means your total caffeine load at bedtime could easily be as high as it was in the morning—especially if you’ve been sipping coffee, tea, or energy drinks throughout the day. And that can be enough to delay the onset of sleep, reduce deep sleep, or cause early-morning awakenings.
I used to feel like having a cup of coffee at 2 or 3 pm was not so bad. After all, it would be another 8 hours before I was trying to sleep. But once I really understood these half-life dynamics, I realized that this was probably too late in the day for that much caffeine, especially for someone who sometimes struggles with sleep.
That said, this is the time of day when energy can lag. So, I want to offer a few alternative ways to give yourself a little mid-afternoon energy boost.
And if you’re not quite ready to go completely cold turkey in the afternoons, consider switching to a lower-caffeine option.
Finally, a word to the wise about a couple of other coffee alternatives that get bandied about. Yerba mate contains both theobromine and caffeine, with caffeine levels comparable to coffee. And guarana is extremely rich in caffeine—sometimes twice as concentrated as coffee beans. So these are probably also beverages to steer clear of in the afternoons.
The truth is, caffeine’s effects last a lot longer than most of us realize. Although it may feel like it’s totally out of your system by 2pm, it’s not–and adding another dose at that point in the day can leave you with quite a bit of caffeine still circulating in your system when you’re trying to fall asleep. But now that you understand how half-life works, you can use that knowledge to time your caffeine more strategically—and experiment with gentler ways to recharge those afternoon batteries.
If you have a question or topic you’d like me to tackle on the podcast, you can email me at nutrition@quickanddirtytips.com. And if you’d like to find out about having me speak at your next in person or virtual event, you can learn more at wellnessworkshere.com
Nutrition Diva is a Quick and Dirty Tips podcast. Holly Hutchings is our Director of Podcasts. Steve Riekeberg is our audio engineer, Morgan Christianson heads up Podcast Operations & Advertising, and Rebekah Sebastian, is our Manager of Marketing and Publicity and Nat Hoopes is our Marketing and Operations Assistant. Thanks to all of them and thanks to you for listening!