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Over 100,000 metric tons
of caffeine are consumed
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around the world every year.
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That's equivalent to the weight
of 14 Eiffel Towers.
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Most of this caffeine is consumed
in coffee and tea,
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but it's also ingested in some sodas,
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chocolate,
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caffeine pills,
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and even beverages labeled decaf.
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Caffeine helps us feel alert,
focused, happy, and energetic,
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even if we haven't had enough sleep.
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But it can also raise our blood pressure,
and make us feel anxious.
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It's the world most widely used drug.
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So how does it keep us awake?
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Caffeine evolved in plants where
it serves a few purposes.
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In high doses, as it's found in the leaves
and seeds of certain species,
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it's toxic to insects.
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But when they consume in lower doses,
as it's found in nectar,
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it can actually help them remember
and revisit flowers.
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In the human body, caffeine acts as a
stimulant for the central nervous system.
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It keeps us awake by blocking one of
the body's key sleep-inducing molecules,
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a substance called adenosine.
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Your body needs a constant supply
of energy,
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which it gets by breaking down
a high-energy molecule called ATP.
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In the process,
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it liberates adenosine,
ATP's chemical backbone.
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Neurons in your brain have receptors
perfectly tailored to this molecule.
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When adenosine docks to this receptors,
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it activates a cascade
of biochemical reactions
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that cause neurons to fire more sluggishly
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and slow the release of important
brain signaling molecules.
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In other words, you get sleepy.
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Caffeine is what's called an adenosine
receptor antagonist.
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That means it derails this process
of slowing your neurons down
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by blocking adenosine receptors.
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Caffeine and adenosine have a similar
molecular structure,
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close enough that caffeine can wedge into
the adenosine receptors,
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but not close enough to activate them.
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To summarize,
adenosine inhibits your neurons.
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Caffeine inhibits the inhibitor,
so it stimulates you.
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Caffeine can also boost positive feelings.
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In some neurons, the adenosine receptors
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are linked to receptors
for another molecule called dopamine.
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One of dopamine's roles in the brain
is to promote feelings of pleasure.
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When adenosine docks in one of these
paired receptors,
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that can make it harder for dopamine
to fit in its own spot,
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interrupting its mood-lifting work.
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But when caffeine takes adenosine's place,
it doesn't have the same effect,
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and dopamine can slide in.
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There's evidence that caffeine's effects
on adenosine and dopamine receptors
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can have long-term benefits, too,
reducing the risk of diseases
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like Parkinson's, Alzheimer's,
and some types of cancer.
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Caffeine can also ramp up the body's
ability to burn fat.
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In fact, some sports organizations think
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that caffeine gives athletes
an unfair advantage
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and have placed limits on its consumption.
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From 1972 until 2004,
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Olympic athletes had to stay below
a certain blood-caffeine concentration
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to compete.
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Of course, not all of caffeine's effects
are so helpful.
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It might make you feel better
and more alert,
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but it can also raise your heart rate
and blood pressure,
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cause increased urination or diarrhea,
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and contribute to insomnia and anxiety.
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Plus, the foods and beverages
caffeine is found in
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have their own impacts on your body
that have to be taken into account.
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Your brain can adapt to regular
consumption of caffeine.
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If your adenosine receptors
are perpetually clogged,
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your body will manufacture extra ones.
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That way, even with caffeine around,
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adenosine can still do its job
of signaling the brain to power down.
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That's why you may find you need
to consume more and more caffeine
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to feel as alert.
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There are more and more adenosine
receptors to block.
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It's also why if you suddenly
quit caffeine,
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you may experience
an unpleasant withdrawal.
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With plenty of receptors
and no competition,
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adenosine can work overtime,
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causing symptoms like headaches,
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tiredness,
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and depressed moods.
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But in a few days, the extra adenosine
receptors will disappear,
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your body will readjust,
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and you'll feel just as alert as ever,
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even without an infusion of the world's
most popular stimulant.