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How Does Caffeine Wake You Up?

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Channel: BrainStuff - HowStuffWorks
Categories: Chemistry   |   Health   |   Science  
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When you get coffee jitters, caffeine has tricked your brain into anticipating danger. Lauren explains how it works.

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According to research from Johns Hopkins University, up to 90% of people in the United States habitually consume caffeine at doses that make it by far America’s most popular drug. A

nd we’re not even the world leaders in coffee consumption per capita: I’m looking at you, Finland and The Netherlands.

Caffeine is a natural compound found in plants like cacao, coffee, and tea, and researchers think that its perky properties have made it part of humanity’s diet since before the dawn of recorded history. So let’s take a look at why it’s so popular – by delving into the chemistry of the human body.

Every cell in your body breaks down a biochemical called adenosine triphosphate, or ATP, to get the energy it needs to do stuff.

Y’know, contract your muscles, transmit electrochemical signals in your brain, build new cells – that kind of thing.

So when your cells do any kind of work, from helping you fight a bear to helping you read (and understand) a probably-harmless book that you found in the basement of a creepy cabin in the woods, those cells wind up excreting a few byproducts from the breakdown of ATP.

One of them is a compound called adenosine. In the brain, adenosine is known to cause drowsiness. When it binds to specialized neural receptors that it fits into like a chemical key into a lock, adenosine slows down (aka inhibits) the activity of particular neurons.

Those neurons happen to be in parts of the basal forebrain that control whether you’re awake or asleep. Adenosine makes those neurons fire less often, and less activity equals drowsy.

Caffeine just happens to fit into the same receptors in the brain as adenosine -- sort of like a lock pick. So part of what’s happening when you drink coffee (or cola or whatever) is that less adenosine can reach those receptors.

Furthermore, where adenosine slows down neural activity, caffeine speeds it up. Those neurons start firing more than usual.

This extra activity leads to caffeine’s physical effects because our brains are dumb. One job that our hypothalamus has is to monitor for extra neural activity because that’s a sign of stress, or heightened arousal, or danger.

Basically, your hypothalamus can’t tell the difference between an espresso and an attacking kaiju, so its response is the same either way. It signals your pituitary gland and your sympathetic nervous system that there’s an emergency, and that they’d better get the adrenal medulla to release some epinephrine, aka adrenaline.

Adrenaline, of course, being the “fight-or-flight” hormone that puts your body on red alert. It makes your pupils dilate. Your airway opens up. Your heart beats faster and your blood pressure rises. Skin-surface blood vessels constrict to slow blood flow from potential cuts and increase flow to muscles. Your liver releases sugar into your bloodstream for extra energy. And your muscles tighten up, ready for action.

In other words, you’re real awake.

The half-life of caffeine in your body is about six hours. Meaning that if you drink a cup of coffee containing about 200 milligrams of caffeine at 3:00pm, you’ll still be left with about 100 milligrams by 9:00pm, and about 50 by 3:00am.

Your brain may still be having trouble binding enough adenosine to really slow down. Thus keeping you awake – or at least, preventing deep, restful sleep.

That’s one reason why experts recommend moderation when it comes to caffeine intake: Less than 300 milligrams per day, which equals about two cups of coffee.


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