Hank: If you’ve ever missed your morning cup of joe and been slammed with a killer headache? You are not alone. Withdrawal symptoms from caffeine, from headaches to fatigue, have been reported in medical literature for nearly two centuries, although most of the peer-reviewed research is more recent.

Fact of the matter is, caffeine is addictive and affects your brain and nervous system. Of course doctors consider it to be pretty benign compared to many other addictive substances. But you can become dependent on your daily coffee fix.

Caffeine works by interfering with a molecule called adenosine. Adenosine is found all over your body. But in your brain, it helps regulate different neurotransmitters and can influence how sleepy you feel.

By binding to receptors on your brain cells over the course of a day, adenosine basically tells your brain to step on the brakes. Because caffeine molecules look pretty darn similar to adenosine molecules, caffeine can bind to the exact same receptors and block their function. So caffeine stops adenosine from doing its thing, while some stimulating neurotransmitters are still around. That’s why you feel alert and awake.

Now, scientists aren’t entirely sure about the biochemistry behind caffeine withdrawal. Some studies suggest that if you have caffeine day in and day out, your brain cells start to make more adenosine receptors to compensate for all the blockage, trying to keep your brain working normally.

So if you take your morning coffee out of the equation, suddenly there are extra receptors available for adenosine to bind to, changing neurotransmitter levels and making you a little sleepier.

Regardless of what’s exactly happening on a molecular level, when the chemistry of your brain changes, it’s a recipe for withdrawal. Around 12 to 24 hours after quitting caffeine, your brain starts experiencing changes in blood flow and electrical activity. Those changes can lead to the headache, drowsiness, and decreased alertness reported by people who get rid of their morning coffee which can last anywhere from two days to a couple weeks.

Sometimes, withdrawal is also influenced by a psychological phenomenon called expectancy. Basically, if you’re aware that withdrawal headaches are a thing, you might expect them to happen and that expectation could influence what symptoms you actually experience.

But a 2004 review of over 50 papers from medical literature found that at least some studies of caffeine withdrawal were sufficiently designed to keep expectancy from affecting their results. So expectations might have some impact on your symptoms, but caffeine withdrawal definitely has a physiological impact too.

In other words, that daily double shot caramel macchiato is totally altering your brain chemistry. But, hey, if it helps you concentrate, there doesn’t seem to be too much harm in it... as long as you’re ok with never stopping.

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