It also affects a host of processes in our bodies, including our digestion, metabolism, and vision.
Here's what's really going on after you drink a cup of joe.
Caffeine is the most commonly used psychoactive drug in the world
One of the things rarely mentioned about caffeine is that it is, in fact, a drug. In fact, it's the most commonly used psychoactive drug in the world, which is probably why we don't think about it as a drug.
Caffeine has psychoactive effects, and changes the way we feel and interact with the world around us. Yet think of how many of us can't - or won't - go through a day without it.
Harvard neuroscientist Charles Czeisler has hypothesised that caffeine, combined with electricity, allowed humans to escape natural patterns of sleep and wakefulness, breaking them free from the cycle of the sun. That change, he wrote in National Geographic, enabled the "great transformation of human economic endeavour from the farm to the factory."
It makes us feel alert, at least for a while
It's normal to grow tired as the day progresses - our brains naturally produce more of a molecule called adenosine from the time we wake up until the time we go to sleep. Scientists think this helps us get to bed at night.
Caffeine hijacks this natural process by mimicking adenosine in the brain. It latches onto the receptors designed for adenosine, pushing them out of the way. As a result, we're left feeling more alert and awake.
Eventually, however, adenosine wises up to caffeine's act and makes new receptors for the sleep-inducing molecule to start latching onto again.
This is why your morning cup of coffee can suddenly turn into two - the more receptors you have, the more caffeine you need to plug them up.
It boosts our mood
As a central nervous system stimulant, caffeine doesn't just boost alertness, it can also improve your mood.
This is due to the same adenosine-blocking effect that makes you feel alert. By blocking adenosine's relaxing effects, caffeine lets dopamine and glutamine (other natural stimulants produced by your brain) run wild, making you more alert, less bored, and providing a mood boost.
Interestingly, a number of studies have found a connection between caffeine consumption and a reduced risk of depression- especially when it's consumed in the form of coffee. Research has even suggested caffeine lowers the risk of suicide, at least for men. However, at least one of these studies only found this connection with caffeinated coffee, not tea, though others found the same effect for tea as well.
However, caffeine also pumps up our adrenaline levels, which can leave us irritable
Caffeine excites our brain cells, which tells our hormone control centre the pituitary gland that there's an emergency. The pituitary tells the adrenal glands (located above the kidneys) to flood the body with adrenaline.
That's the hormone behind the "fight or flight" response. Adrenaline prompts us to either stay and face a threatening situation or flee a scene. In this excited state, we tend to be more irritable, anxious, and far more emotionally charged.
While that can be helpful for running from someone or defending ourselves in a fight, the aggressive hormone does little good in more delicate situations like negotiating in a meeting or responding to a text.
It improves our memory
Caffeine has been shown to improve certain types of memory - especially the ability to remember lists of words and straightforward information - in some (but not all) studies. Some research shows that it helps those memories "stick" in the brain as well, making it easier to recall that information later.
This enhancement, however, seems to be strongest for people who aren't already hooked on caffeine in the first place.
One recent study also indicates that extroverts get more of a working-memory boost from caffeine than introverts. This may explain why some studies have observed a more significant effect than others. Stephen Braun, the author of Buzz: The Science and Lore of Alcohol and Caffeine, explains that individual's reactions to caffeine vary greatly - while one person might thrive on a high level of caffeine, it'll make another person unable to get anything done.
Too much caffeine can lead to a decrease in performance across the board, however.
Caffeine can increase our attention span
One of the common reasons people drink caffeinated coffee or tea is to help them focus on a task, and no wonder: one of the clearest mental effects of caffeine is a boost in the ability to focus, especially for someone who is fatigued.
Research shows that commercial drivers who cover long distances are significantly less likely to crash if they've consumed caffeine in any form - coffee, tea, pills, or energy drinks.
However, most people are familiar with caffeine jitters too, and know it can get hard to focus on anything after consuming too much.
But some of caffeine's benefits may come from the fact that it's hooked us
Some research suggests that caffeine's perceived benefits aren't really benefits at all.
For some, these studies suggest, all of the positive effects of caffeine - from better mood to improved memory and attention span - may be the result of a dose of caffeine temporarily reversing the effects of longer-term withdrawal from the drug.
In other words, when someone who's hooked on coffee stops drinking it, going without the drink might make them feel tired and less attentive. When they start drinking it again, their performance may only increase because the brain and body had already become addicted to caffeine.
It might tamp down your appetite - but only for a short while
A cup of coffee likely blunts your appetite for a brief time, but there's little to no evidence that making caffeine a regular habit can keep hunger pangs at bay or help with weight loss.
Most studies looking at caffeine's effect on appetite have either been too small or only done in animals, making it hard to say whether the researchers' observations would apply to people more broadly.
Caffeine helps some medicines work faster
If you've ever had a killer migraine, you've likely tried Excedrin, an over-the-counter medication marketed for these types of rare, severe headaches. In addition to traditional pain relieving ingredients like ibuprofen and acetaminophen, Excedrin contains caffeine.
That's because there's some evidence that caffeine helps certain pain-relieving medications, like acetaminophen (the main active ingredient in Tylenol) and aspirin, take effect quicker, last longer, and be more effective.
A 2007 study of 24 people who took either a combination of caffeine and acetaminophen, just one or the other, or a placebo found that those who'd taken both together saw a stronger decrease in pain symptoms - an effect that also tended to last longer.
It's one of the best athletic performance enhancers out there
Caffeine is one of the most common performance-enhancing drugs used in sports.
"If you can tolerate it, it seems to be the upper end of what you can have to improve performance," exercise physiologist Matthew Ganio told The Atlantic.
If dosed correctly (and assuming it isn't just returning a caffeine addict to a baseline level), it can give athletes notable performance gains - as long as they don't use too much of it in day-to-day life.
Caffeine sticks around in our bodies for a long time
Ever wonder if that late afternoon cup of coffee (or evening espresso) is going to keep you up at night?
It takes about 5 or 6 hours before half of the caffeine you've ingested wears off - so a cup of coffee at 4 p.m. could leave you still feeling half of its effects by 10 p.m.
If that's enough to keep you up, plan accordingly.
Caffeine can give you heartburn
While a warm cup of coffee might seem soothing if you're cold or tired, caffeine also raises acid levels in your stomach. This can lead to heartburn and can be especially bad if you suffer from ulcers.
Regular coffee consumption is associated with a longer lifespan
Two recent, major studies have found evidence that people who drink a lot of coffee are less likely to die an early death.
Researchers who've analysed the diet and health of hundreds of thousands of individuals have suggested that heavy coffee drinkers are less likely to die from heart disease, cancer, and diabetes, among other conditions.
However, decaf coffee seems to convey many of these same health benefits - so even if you are looking to cut down on caffeine, you shouldn't necessarily end your coffee habit.
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