Jen Oliver: Welcome to the, so that's Why podcast, where we unpack the
science behind everyday questions.
I'm Jen.
Chris Smith: I'm Chris.
Jamie Eynon: and, I'm Jamie.
Jen Oliver: So tell me if this sounds familiar.
You used to drink caffeine and feel like you could conquer the day, but
now you drink it just to start feeling like you're running on a low battery.
Why does caffeine lose its magic?
Jamie Eynon: Yeah, and it's especially noticeable with coffee.
There's a full on arms race going on.
You start with a single shot latte.
The next thing you know you're ordering a quadruple espresso
and the barista is looking at you like you need an intervention.
Chris Smith: Totally, and it probably won't surprise you that nearly 90%
of adults consume caffeine daily.
Um, it's the most widely used psychoactive substance on the planet,
and yet most people have no idea why it gradually loses its punch.
Jamie Eynon: And before we get into Jen, you are more of a tea person.
Aren't you
Jen Oliver: Guilty as charged?
I'm a solid four cups of tea a day person, but I've noticed the same thing.
You know, that first Brew of the morning used to give me like a proper lift,
but now it just feels like maintenance.
Chris Smith: That's because the mechanism is identical, whether it's coffee,
tea, energy, drinks, or cola, the molecule, doing the work is the same.
Caffeine tea has less per cup, typically, uh, 30 to 50 milligrams compared to about
95 milligrams in coffee, but your brain responds to it in exactly the same way.
So today we're talking about all caffeine, not just from coffee.
Jamie Eynon: Plus we'll get into why some people can drink espresso at 9:00
PM and sleep soundly while others are wide from a lunchtime cup, whether
you can actually do anything about it.
Jen Oliver: So what's really going on here, because most
people think caffeine tolerance, it's just a minor inconvenience.
Chris Smith: They do, but it is much more than that.
In fact, a 2017 trial found that while caffeine still improved
mental and physical performance after two weeks of daily use.
Those benefits completely disappeared after one month of daily intake,
Jamie Eynon: and that's a big deal though, isn't it?
If you are relying on a cup of coffee or tea to get through a busy morning
or power through a deadline, and it's basically doing nothing after a few weeks.
You're just going through the motions.
Jen Oliver: So the effects completely disappear after one month.
So every January when I set my, you know, new, new year, new me goals, my caffeine's
already checked out before I have,
Chris Smith: I know, depression.
Right?
And here's where the part that like really gets people, because
research shows that once that full tolerance develops habitual caffeine.
Drinkers are essentially consuming it just to return to the baseline
they had before they started.
So without that morning cup, they'd actually feel worse than someone who'd
never touched the caffeine at all.
Jamie Eynon: So we're all locked in a cycle where our brew stops giving us
a boost and just stops us feeling, or just stops us feeling terrible instead.
Jen Oliver: I mean, it's a pretty sobering thought for something we all
reach for first thing in the morning.
But Chris, what is actually going on in our brains to cause this?
Chris Smith: So to understand caffeine tolerance, we need to start
with a molecule called adenosine.
So throughout the day, your body produces this adenosine as a
natural byproduct of burning energy.
Adenosine binds to specific receptors in your brain, mainly called
the A one and A two A receptors.
And when it does, those receptors send you the signal you are tired, slow down.
Jen Oliver: So adenosine is essentially your brain's built-in brake pedal,
the longer you're awake and active.
The more adenosine accumulates and the stronger that signal
to be to rest becomes,
Chris Smith: that's it.
But what gets really interesting is that caffeine is structurally
almost identical to adenosine.
So when you drink coffee or tea, the caffeine molecules
slot into those same receptors.
But they don't activate them.
They just sit there blocking, adenosine from getting through.
Jamie Eynon: So the tired message is still being sent.
Caffeine is just stopping it from being delivered.
Chris Smith: Exactly right.
So think of it as like noise canceling headphones for your
brain's tiredness signals.
The noise is still there, but you just can't hear them.
Jen Oliver: Which is a really important distinction because that
means caffeine doesn't actually create energy or alertness.
It's just blocking your ability to feel tired.
So the adenosine is still building up in the background the whole time.
Chris Smith: Yeah, precisely.
And this is really the key to everything.
So your brain notices that it's tired signals.
Aren't getting through.
So it adapts by becoming more sensitive to them.
In fact, it does this by growing more adenosine receptors to compensate.
And a landmark study found that regular caffeine users increased adenosine
receptors in the brain by about 20 to 30%.
And another study showed that about 400 milligrams of caffeine per day, which
is roughly four cups of coffee for just 14 days, was enough to significantly
increase those receptors in humans.
Jamie Eynon: So your brain is basically hiring extra staff to handle the
complaints your coffee keeps ignoring.
Chris Smith: Yeah.
That's a great way of putting it.
And once those extra receptors are in place.
You now need more caffeine of course, to block a greater number of them.
The dose that once made you feel sharp now barely gets you back to normal.
Jen Oliver: And how quickly does this all happen?
Because it feels like a gradual thing.
But I can imagine the biology probably kicks in a lot faster than we'd expect
Chris Smith: it is it?
It's much faster once you read up it.
It's quite surprising.
In fact, one study found that complete tolerance to caffeine's
effects on blood pressure and heart rate occurred after just one to
four days of regular consumption.
The alertness and, uh, focus benefits last a bit longer, but a 2024 review confirmed
that measurable receptor changes occur within two weeks at moderate intake doses.
Jen Oliver: So within a fortnight, your brain's already restructured itself
to counteract your caffeine habit.
I mean, that's incredibly fast.
Jamie Eynon: And this explains the crash too, doesn't it?
Because I've noticed if I skip my afternoon coffee, I feel worse than if
I'd never had a coffee at all that day.
Jen Oliver: Mm-hmm.
Chris Smith: Yeah, that is spot on.
And it's one of the most counterintuitive parts.
'cause while caffeine, these occupying those receptors we talked
about, adenosine keeps accumulating.
So once the caffeine wears off and it's half-life is about.
Four to six hours, all that built up adenosine floods in at once.
So for chronic users, it's even worse because you've now got more
receptors than you started with.
So the tiredness signals hit harder than it would for a non caffeine drinker.
Jen Oliver: So habitual caffeine drinkers actually experience more fatigue without
it than someone who never drinks it.
I mean, that's quite the trade off.
Chris Smith: I know.
Depressing isn't it?
Um, a researcher described caffeine as a alone of the awake feeling rather
than the creation of any new energy.
So at some point you have to pay it back with interest.
Jamie Eynon: Caffeine is basically a payday loan for your brain.
Good short term.
But the repayment plan is rough.
Jen Oliver: And the twist is that not everyone gets the same repayment plan.
Genetics play a big role in how caffeine affects different people.
Chris Smith: That's right.
And twin studies suggest it that genetics account for about 36 to,
I think 58% of the differences in how people respond to caffeine.
And there are two key genes involved here.
The first one is called the CYP one A two, which produces the enzyme
responsible for metabolizing about 95% of the caffeine in your body.
And a small variation in this gene can determine whether you
are a fast or a slow metabolizer.
So fast metabolizers down caffeine about one and a half or 1.6 times
faster than a slow metabolizer.
Jamie Eynon: So that friend who swears they can have an espresso
after dinner and sleep fine.
They're not making it up.
Chris Smith: No, not at all.
And about 10% of the population carry a genetic variant, um, linked to
higher caffeine tolerance, and that allows them to consume larger amounts
later in the day without side effects.
The second gene called adora two A affects the adenosine receptor itself.
Different variations determine whether caffeine is more likely to keep you
awake or more likely to make you anxious.
Jen Oliver: So your entire relationship with caffeine, how much it wakes you
up, how quickly you build tolerance, whether it keeps you up at night,
is heavily influenced by your DNA.
So that explains why some people can drink caffeine late in the
day and feel fine while others are wired for hours after just one
cup in the afternoon, which is me.
That is me.
Jamie Eynon: This explains every argument in every office, kitchen, ever.
How can you drink at 4:00 PM How can you not?
Turns out we're all just having a genetics debate and we didn't know it.
Chris Smith: Totally.
And there's a myth worth addressing here.
I'm always the myth buster.
Jen Oliver: You are the Myth Buster
or Captain Sensible.
Chris Smith: I know so many people believe they've successfully adapted
to less sleep by using caffeine, but multiple studies show that while
you might feel less acutely tired.
The cognitive performance continually deteriorates.
So your brain is actually becoming numb to its own impairment.
Jen Oliver: So we touched on this in our sleep episode, the idea that
people can accumulate sleep debt without realizing how impaired they
are, and then caffeine masks that even further, which is why it's such a trap.
Jamie Eynon: Uh, what about age?
I definitely feel like coffee hits differently now compared
to how it did in my twenties.
Chris Smith: No, absolutely.
So caffeine, metabolism slows with age, meaning it stays
in your system for longer.
This is why many people find they can't tolerate a late afternoon
coffee as they get older.
They're not just imagining it, so their liver is simply processing it more slowly,
Jen Oliver: which explains why some people naturally switch to lower
caffeine like decaf tea over time, because lower caffeine per cup means
less impact on your sleep, especially if your metabolism slows with age.
Jamie Eynon: Alright, so the big question we actually, can we
actually do anything about this?
Or are we stuck in an endless cycle?
Chris Smith: Yeah, you can absolutely manage it.
And the most effective approach is strategic use of caffeine rather
than habitual use of caffeine.
So research shows that using caffeine on only days when you genuinely need
peak performance, let's say two to three days a week, prevents that steady
receptor buildup that causes tolerance.
Jen Oliver: And for people who wanna reset their tolerance, studies show that.
Brain receptor sensitivity.
It typically normalizes within about two weeks of stopping caffeine.
After a two week break, your sensitivity returns roughly to 70, 80% of its
original level, but heavy users obviously might need, you know, up
to two full months for a full reset,
Jamie Eynon: two weeks without coffee.
I think most people would rather live.
With weaker caffeine effects than deal with two weeks with no coffee.
Jen Oliver: Definitely.
Chris Smith: Uh, yeah, I think that's fair.
And there is a gentler option you can take.
So gradually reducing that caffeine intake by about 25% every 10 days,
that will produce significantly fewer withdrawal symptoms.
And research shows that about 80% fewer severe effects compared
to stopping abruptly for people.
Jen Oliver: It is also worth mentioning that caffeine withdrawal
is actually clinically recognized.
It's no joke is it?
You know, people experience symptoms such as headaches, fatigue,
irritability, difficulty concentrating, and those symptoms typically peak
a few days after, but then they do resolve within a couple of weeks.
Jamie Eynon: And that applies to tea drinkers too?
Chris Smith: It does, although tea drinkers often have a smoother
experience 'cause their daily caffeine intake tends to be lower.
If like Jen, you're on about four cups of tea a day, you're looking at maybe 150
to 200 milligrams of caffeine in total.
If you compare that to the same number of coffees like four a day,
that's closer to 400 milligrams.
So the withdrawal is proportionate to the intake.
Jen Oliver: I mean, that's reassuring.
So tea drinkers like myself might build tolerance more slowly.
And have a gentler time.
Resetting
Chris Smith: generally yes, and keeping daily intake below roughly 200
milligrams, which is about two standard coffees or forties that slows the
rate at which that tolerance builds.
And one study found that 400 milligrams of caffeine daily for only seven
days, had no measurable receptor effect, but the same dose for 14 days.
Did.
So moderate tea drinking may sit in that sweet spot where you're getting
some benefit without triggering those major receptor changes.
Jamie Eynon: So it's about finding that balance where you are getting the
benefit without tipping into the zone where your brain starts remodeling.
Jen Oliver: And here's something else that's really interesting.
For anyone who exercises, caffeine still appears to boost physical
performance, even intolerant individuals.
And a study showed that caffeine supplements, they enhance physical
performance by an average of around 11%.
So those that are habitual uses, they might need a higher dose, but the
benefit doesn't disappear entirely the way the mental alertness benefit does.
Chris Smith: No.
And 11% is huge, isn't it?
Jen Oliver: Mm-hmm.
Chris Smith: But the catch is that caffeine improves performance.
But can disrupt sleep.
And of course poor sleep undermines recovery.
So for anyone exercising regularly, the timing matters.
Ideally, no caffeine within about eight hours of bedtime.
Jamie Eynon: That's really interesting because.
It creates a real tension.
You want the performance boost, but not at the cost of your sleep.
Jen Oliver: And I think, uh, you know, most of us have probably been
in that cycle where we drink the caffeine to compensate for poor sleep.
Then the caffeine disrupts our sleep, so then we need more the next day.
It's, it is the definition of a vicious circle.
Chris Smith: Yeah, it really is.
And, and it connects directly to what we discussed in previous episodes.
So sleep quality affects everything.
Mood.
Cognitive function, uh, exercise recovery, even how your body manages inflammation
and caffeine tolerance sits right in the middle of all these systems.
Jamie Eynon: Uh, and hydration too, right?
If you're drinking four or five cups of coffee a day or tea because one
doesn't cut it anymore, that's a lot of fluid moving through your system.
Jen Oliver: Yeah, that's a good point.
At normal consumption levels, coffee and tea are both hydrating and we
covered that in our hydration episodes.
But when tolerance pushes intake higher, it's worth being aware
of your overall fluid balance.
Chris Smith: Yeah, totally.
And what is really striking is that caffeine tolerance is a
perfect example of how our bodies constantly adapt to maintain balance.
And we saw the same principle with sweat acclimatization, with sleep cycles,
even with cholesterol regulation, the body's always adjusting and adapting.
Jamie Eynon: So understanding this isn't just about coffee, it's about
us understanding how your body works.
Full stop.
Jen Oliver: So why does your morning coffee or tea stop working?
Chris Smith: It stops working because your brain physically adapts to
caffeine by growing more adenosine receptors, sometimes 20 to 30% more.
The dose that once blocked enough receptors to make you feel alert now
can't keep up with increased numbers.
You end up drinking caffeine just to feel the way you would naturally without it.
Jamie Eynon: So we went from one cup making us feel super human to four
cups making us feel merely human.
And we've been calling that a coffee habit when it's actually
a brain renovation project.
Jen Oliver: So that's why caffeine stops working, and now you know that your
relationship with it is shaped by your genetics, your habits, and your brain's
relentless drive to maintain balance.
But the good news is once you understand the mechanisms, you can
work with it rather than against it.
Chris Smith: You can.
And the advice is to use caffeine strategically, not habitually.
So a periodic reset or simply cutting back can restore much of that original effect.
Sometimes less really is more.
Jen Oliver: So that's it for this week.
Make sure you hit that subscribe button.
We're unpacking the science behind everyday questions each week,
and tell us in the comments what questions you'd like us to explore.
Thanks for geeking out with us, but for now, that's it from me.
Jamie Eynon: That's it from me.
Chris Smith: And that's it from me.
Jen Oliver: Bye for now.