Wellness·4 min read

The Unexpected Side Effects of Caffeine — Including Why Caffeine Makes You Tired

A plastic cup of iced coffee with milk
Design: theSkimm | Photo: Pexels
July 19, 2022

It’s a daily staple in many people’s lives, but there’s still a lot of mystery surrounding caffeine. Like, why does caffeine make you sleepy? And why can it be both a remedy for headaches and a trigger? 

We dug into these big Qs with the help of Jessica Kruger, clinical assistant professor of the Department of Community Health and Health Behavior at the University at Buffalo, who researches addictive behaviors, including those related to caffeine.

What are the side effects of caffeine? 

If you drink more than the daily recommended amount of caffeine, which is 400 milligrams (think: four cups of coffee or five cans of Red Bulls), Kruger says you might experience side effects, including…

Which brings us to…

Why does caffeine make me tired?

First, a quick bio lesson: Feeling sleepy, in general, is caused in large part by adenosine — a chemical that helps regulate our sleep-wake cycle. The amount of adenosine in the body increases the longer you stay awake (causing you to feel tired) and decreases while you sleep. And the cycle repeats itself each day.

Enter: Caffeine, which has a similar structure to adenosine. So when you drink coffee or an energy drink, the caffeine temporarily blocks adenosine from doing its job (read: making you sleepy). Which gives you that feeling of alertness. 

But, plot twist: that doesn’t actually stop adenosine from being produced. It just builds up. Along with your need to sleep (altogether what scientists call "sleep pressure.”) Then when the caffeine effects wear off, all the built-up sleep pressure is still there. Making you feel even more tired. 

The only real way to stop feeling sleepy, with or without caffeine? Getting more sleep. Read: taking a nap (if you’re able) instead of downing that 3pm cup of joe might be a better and more effective alternative to boost your energy levels. 

Another reason why caffeine might make you feel tired: dehydration. Caffeine is a diuretic. Meaning it rids your body of water, and makes you need to use the bathroom more, which can cause dehydration…and can also make you sleepy. So much for the energy boost you were looking for.

Why can caffeine cause headaches — but also sometimes alleviate them?

Here’s another caffeine oxymoron for you: It can be both the cause of, and a temporary pain reliever for, headaches. Why it causes headaches: withdrawal symptoms. 

So if you decide to give up caffeine and “you are used to having caffeine every morning, you're going to get a headache,” Kruger said. And even if you haven’t given it up entirely, Kruger said that your body might start to feel the withdrawal effects if you have your daily coffee later than usual. So if you sleep in late, you might wake up with a headache

Caffeine narrows the blood vessels around the brain. When you stop drinking caffeinated drinks, those blood vessels enlarge, causing the blood flow and pressure on the nerves to increase. Which can give you a headache

But, because nothing is as simple as it seems, caffeine can also help headaches. Blood vessels tend to enlarge before a headache, which causes the pain, and since caffeine causes the blood vessels to narrow, it can temporarily alleviate that pain. Which is why you’ll see caffeine as one of the ingredients in some headache medications (think: Excedrin Migraine or Aspirin). 


While caffeine’s main “purpose” is to wake you up, it can often make you feel tired too. That might be because you need more sleep. So instead of making another pot of coffee, why not try an afternoon nap instead?

This content is for informational and educational purposes only. It does not constitute a medical opinion, medical advice, or diagnosis or treatment of any particular condition. 

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