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How Does Coffee Wake You Up

Written by Phil

A coffee enthusiast and traveler, Phil primarily spends his days thinking and writing about coffee. He also writes for HittingEject.com where he explores ideas of culture, psychology, and travel, and occasionally dabbles in horror.

In the US, 90% of people drink at least one caffeinated beverage per day. I don’t know about you, but that sounds slightly low to me. 

Caffeine is the most widely used psychoactive substance in the world and seems to only grow in popularity as time passes. But many people are unaware of how caffeine adds some pep to their step. We’re left wondering: how does coffee wake you up?

Caffeine blocks adenosine slots in your brain. Adenosine (a chemical that makes you tired) builds up, but you don’t feel it until your body clears the caffeine. Until then, you will feel alert, focused, and energized.

Why do we feel tired?

As Matthew Walker PhD eloquently states in his book, Why We Sleep, the brain has a number of chemicals floating around in it that have various effects, one of which is adenosine. 

Adenosine can make you feel tired. As it builds up, you will feel more and more “sleep pressure”, as scientists call it. In layman’s terms, you feel sleepy.

Adenosine is a result of our brain using another molecule for energy (among other things), called ATP (Adenosine Triphosphate). Once processed, adenosine is a byproduct and will continue to build up until you sleep. 

Neurochemistry wasn’t my best subject in school, but this should be generally accurate.

How does caffeine make you more alert?

The brain is a semi-isolated system in the body and is protected by something called the “blood-brain barrier”, which keeps out stuff that might be dangerous.

Caffeine is something that is able to pass this barrier, directly interacting with the brain.

The brain itself can be seen, if we reduce it to a caricature, as a limited bunch of lock-and-key-like sockets. Adenosine and caffeine are similarly-shaped keys.

Think of it like a building that was built with electrical outlets from all over the world. If all the European-style ones are being used, then any European waiting to charge their phone will have to wait. So it is with caffeine and adenosine.

To simplify as much as possible: caffeine creates a dam in your brain that holds back adenosine. As long as caffeine is there, the dam remains – but adenosine continues to build up. Eventually, the dam will break when your body disposes of the caffeine, allowing the adenosine to bear down on you like a wave. Thus, the caffeine crash.

Can you overdose on caffeine?

You can overdose on water (look up water toxemia), so of course, you can overdose on caffeine. As they say, the dose makes the poison.

In the case of caffeine, the general guideline is that 400mg of caffeine per day is considered safe (source). This will vary depending on body composition, body mass, gender, and several other factors.

Overshoot what your body can handle, and you may start to suffer from:

  • Migraines
  • Muscle Tremors
  • Insomnia (shocking)
  • Nervousness

Though, if you take enough, you can definitely die. Apparently, 1 tablespoon (14.2g) of pure caffeine is the equivalent to 75 cups of coffee. If you were to take this, your heart would probably stop. 

This seems an outlandish consideration until you realize that more and more “pure caffeine” products are being introduced to the market, including caffeine gum and caffeine pills. At least 2 healthy adults have died in the US (as of 2018) from overdosing on caffeine; I lost the source for this, but they apparently were taking caffeine pills, and they were the reason regulation changed for what was allowed on the market.

It’s unlikely, but you should be aware of the risks. So long as you’re drinking regular coffee, even in its most condensed forms, it’s unlikely you’ll ever reach the concentration that synthetic/isolated caffeine can bring.

Why can some people fall right asleep after drinking coffee?

This is a trickier one. There are a number of reasons I came across, some more valid than others:

  • Dehydration: While I don’t think this is a huge factor (coffee doesn’t seem to dehydrate much), if you don’t drink much water, then you may feel tired from a lack of water. Again, probably not too relevant.
  • Sugar crash: Some have said that fatigue is the result of your body clearing whatever sugar you added to your coffee, including sugar, honey, and sweeteners. As you consume these sweet substances, your body releases insulin, which then clears it out of your bloodstream, leading to a lack of energy. Likely a more chronic problem.
  • Mold: A 2003 study said that 91.7% of green coffee samples from Brazil were contaminated with mold. They found levels of mycotoxins (myco means fungi), but were deemed acceptable. A 2013 study found that these toxins could lead to chronic fatigue. If you’re particularly susceptible, then that could be an issue. It’s unclear how roasting will affect these toxins, but it’s fairly safe to assume that it will kill the fungi and potentially render the toxins inert. Unclear, but likely not the main reason for people who can drink coffee and go to bed after.
  • Too much adenosine: As mentioned previously, just because coffee blocks the feeling of tiredness doesn’t mean the adenosine isn’t there. I’m going to jump make an educated guess that, if you’re drinking coffee all day, then maybe the amount of adenosine reaches a higher average level. If this were the case, then you may still be able to sleep despite having just drank caffeine.

Aside from these reasons, I couldn’t find anything very scientific. The solutions that were proposed by Healthline were: drink more water, consume less sugar, and don’t drink more than 400mg of caffeine per day (roughly 94.8mg in an 8oz cup, which means ~4 cups or 32oz).

How does it affect sleep?

Caffeine has a half-life of 5-6 hours. If you drink 100mg of caffeine at 6 am, at noon you’ll have 50mg in your system, and at 6 pm you’ll have 25mg. It just cuts in half the current value.

This means that if you drink anything after roughly 2pm, you’ll likely have enough in your system to lower the quality of your sleep, thus creating a vicious cycle of caffeine dependence. Tired? More coffee!

I’m going to go on a bit of a tangent to talk, briefly, about why sleep is important: sleep debt is not a thing. Our bodies never developed a safety net of stored-up sleep to protect our brains and bodies when we don’t get enough.

When it comes to food, our body has a system of storage in case we are starving: fat. Eat too much, store fat. Not enough to eat? Burn the stored fat. This is because food often works in a feast/famine cycle in the lives of animals (including our ancestors).

However, there never were consistent reasons to be lacking sleep. It’s not like there would be long periods for sleep deprivation, so no biological system was made to protect against its lack.

And now here we are, the smartest creatures on the planet, and we’re deciding to forego sleep for… reasons.

This study goes over the short- and long-term effects of sleep deprivation:

  • “Short-term consequences of sleep disruption include increased stress responsivity, somatic [body] pain, reduced quality of life, emotional distress and mood disorders, and cognitive [thinking], memory, and performance deficits.”
  • “Long-term consequences of sleep disruption in otherwise healthy individuals include hypertension, dyslipidemia [fat in your blood], cardiovascular disease, weight-related issues, metabolic syndrome [precursor to diabetes], type-2 diabetes mellitus, and colorectal cancer.”

TL;DR 

Sleep deprivation results in damage to your brain, genes, and overall health. There is no sleep debt that you can pay back – you merely damage the system and move on.

How long does it stay in your system?

As previously mentioned, Caffeine’s half-life is roughly 5-6 hours on average. That means it halves the amount after every cycle.

Like so:

  • 6am: Drink 100mg of caffeine
  • Noon: 50mg left
  • 6pm; 25mg left
  • Midnight: 12.5mg left

It’s possible, if you drink coffee in the afternoon, that you still have caffeine in your system when you reach for your morning Joe.

Fun Facts

It’s theorized that caffeine was first developed in plants because it can be toxic to insects that try to consume the plant. However, it is beneficial when consumed in lower doses, such as through nectar, which can help the insects (namely bees) to remember and revisit, helping the plant to spread its genes.

TL;DR

Your brain uses ATP for energy and motivation, which turns into adenosine when spent. This adenosine builds up and creates “sleep pressure” in your brain.


Caffeine is shaped like adenosine and can block adenosine from registering in the brain. Due to this, your brain doesn’t realize that it’s tired, but the adenosine continues to build up behind the dam of caffeine. 

Caffeine has a half-life of 5-6 hours, which means 50% will be taken away every 5-6 hours. It takes a long time to clear completely since it’s always cutting in half. Once the caffeine is cleared enough, you will feel the effects of the built-up adenosine.

In short: caffeine tricks your brain into thinking you’re not tired by “plugging your ears” and going “la la la” while the music continues to get louder and louder. Eventually, you can’t ignore the outside noise.

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