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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 it 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 these 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.