What Really Happens During a Hangover

The surprising science of the morning after and why it is more than dehydration

It had been a lovely evening. Good conversation, laughter, a few glasses more than you’d planned. And then the next morning: a pounding head, a dry mouth, nausea, and the feeling that you’ve been put through a washing machine.

We call it a hangover, and almost everyone knows it. But what is actually happening in your body? What causes that pain, that malaise, that all-over sense of misery?

The answer is more surprising than you’d think. Because although the hangover is one of the most common forms of headache in the world, the exact mechanism still has not been fully worked out. Scientists even have a grand name for it: veisalgia cephalgia.¹ Behind that name a great deal of mystery remains. And it is certainly not a story of one single cause. Let’s walk through the science, from the familiar to the surprising.

1. The poison your body makes itself

This is where it begins. When you drink alcohol, your liver breaks it down in two steps.

In the first step, an enzyme converts the alcohol into a substance called acetaldehyde. And here is the treacherous part: acetaldehyde is far more toxic than the alcohol itself. It causes flushing, nausea, a racing heart and headache. In the second step, another enzyme converts that toxic substance into harmless acetate, which your body can safely clear away.²

The problem is that these enzymes work at a fixed speed. If you drink faster than your liver can process, the alcohol builds up in your body. And that speed turns out to matter: people who clear alcohol quickly have, on average, a milder hangover than people who clear it slowly.²

This also explains a puzzle you may recognise: why one person is already ill after two glasses, while another can take much more. That is not a matter of “being able to hold it” or strength of character. Inherited make-up is one of the factors that shape how severe a hangover becomes.⁵ It is biology, not willpower.

2. The surprise: maybe it is not what we thought

Here it becomes scientifically fascinating, and it shows beautifully how honest research works.

For a long time scientists thought the accumulated acetaldehyde was the direct culprit. A logical story. But when researchers measured it, they found something unexpected: the amount of acetaldehyde in the blood did not track how bad the hangover was. The amount of alcohol itself did.² In other words: the simple story did not hold.

An animal study pointed in another direction. Not the acetaldehyde itself, but the acetate, the substance from the second step, the very one we considered “harmless”, turned out to increase sensitivity to pain. And once the formation of acetate got going, another substance built up alongside it: adenosine.³

If you have read my earlier articles about sleep, a bell might be ringing now. Adenosine is the same “sleepiness substance” that builds up in your brain during the day. And here is the lovely part: when the researchers blocked the action of adenosine in the rats, the heightened sensitivity disappeared.³ An interesting detail, caffeine also blocks adenosine. That could be a clue as to why a cup of coffee eases the hangover headache for some people.

Important, to stay honest: this was research in rats. For humans we do not know it for certain. But it shows that real science is always more nuanced than the simple story we so often hear.

3. Dehydration, and why it explains less than you think

Everyone knows it: alcohol dries you out. But this is exactly the point where the familiar story starts to creak.

The classic explanation goes like this. Alcohol suppresses a hormone called vasopressin. This hormone normally tells your kidneys: “hold on to water.” When it is suppressed, your kidneys hold on to less water and you pass more urine than usual.⁴ That is why you go to the toilet so often during an evening of drinking.

And here the nuance belongs straight away, because the measurements are messier than the textbook. In a controlled study in healthy men, vasopressin in the blood during that extra urine production was not lower than on an evening without alcohol. Later in the night it was actually higher, and the body held water back.⁴ That you pass more urine is settled. Exactly why is less settled than we think.

You have probably read somewhere that the loss of fluid makes the membranes around your brain shrink and tug at pain-sensitive areas. It is a beautiful image. I simply could not find research in humans to support it, so I will not present it here as fact.

What has been measured matters more. Researchers looked in blood and urine for the markers that betray dehydration, vasopressin included, and found no relationship with how bad the hangover was.⁵ Dehydration probably explains your thirst and your dry mouth. But if it were the whole explanation, a few glasses of water would cure the hangover. And that, as you probably know from experience, it does not.

4. Why red wine and whisky are worse

This explains something you may have noticed: one drink gives a much worse hangover than another.

During the making and ageing of drinks, small amounts of by-products form that are called congeners. They give colour, smell and taste. Dark drinks are full of them: red wine, whisky, cognac, bourbon. Clear drinks such as vodka and gin contain far fewer.

Your body has to process those congeners too. In a study where healthy volunteers drank bourbon on one night and vodka on another, to the same blood alcohol level, they felt worse after the bourbon.⁶ Note the precise result, because it is more modest than you might hope: the congeners made a difference only to how rotten people felt, not to how poorly they performed the next day.⁶ Congeners belong to the factors that make a hangover worse, not to the factors that cause it.⁵

And then red wine, which many people blame on histamine. Here the evidence is honestly divided. In a provocation test, people with wine intolerance developed symptoms after a single glass of red wine, with measurably more histamine in their blood.⁷ But when another group of researchers ran a double-blind comparison of a histamine-rich against a histamine-poor wine in people with the same complaints, the histamine content made no difference at all.⁸ Something in wine does something to some people. Whether that something is histamine itself, we do not know.

5. Why you feel fluish

A newer insight explains why a hangover is so much more than just a headache. A hangover seems partly to be a mild inflammatory response. Your immune system reacts to the alcohol by producing inflammatory substances, the same substances that, during flu, create that rotten, listless feeling. In blood samples, three of those substances tracked the severity of the hangover.⁹

That explains why a hangover affects your whole being, not just your head. The trouble concentrating, the low mood, the irritability, the feeling that the world is too bright and too loud. You feel fluish because your body is in a mild inflammatory state. It is as though you are going through a light, self-inflicted flu.

And here the nuance belongs straight away. That the immune system responds has been shown more than once. But in a study where the same researchers looked at saliva instead of blood, the inflammatory substances they measured did not track how rotten people felt.¹⁰ The response is real. The road from that response to your misery is not yet nailed down.

6. And then poor sleep piles on top

Here one last layer is added, and anyone who has read my articles on sleep will recognise it at once.

Alcohol disturbs your sleep considerably. You may fall asleep faster because of it. That is true. But the quality suffers, especially in the second half of the night. Your REM sleep in particular, the dreaming phase that is important for processing emotions, starts later and takes up less of the whole night.¹¹ In the bourbon and vodka study, participants also slept less efficiently after alcohol and woke more often.⁶

And here too a crack, because it would be too neat to pin everything on sleep: in that same study, the poorer sleep did not explain why people performed worse the following day.⁶ So there is more to it than one bad night.

So the hangover headache becomes a pile-up: the alcohol still being broken down, the disturbed fluid balance, the mild inflammation, and a poor night’s sleep, all at once, all on the same morning. No wonder you feel so rotten. It is not one but five or six things tugging at you at the same time.

The common thread: a body recovering

When you survey all the mechanisms, you see something beautiful. A hangover is in essence your body knocked out of balance and working hard to return to equilibrium. It clears the alcohol. It restores the fluid balance. It puts out the inflammation. It processes the missed sleep.

The headache and the malaise are, from that point of view, not a punishment. They are the sound of a body tidying up after a disruption. It is recovery, and recovery rarely feels pleasant while it is happening.

And there is an honest scientific truth I do not want to keep from you: there is no proven miracle cure for a hangover. No pill, no drink, no breakfast that truly cures it. A systematic review of randomised trials examined eight remedies, from propranolol to artichoke extract, and found convincing evidence for none of them.¹² The only thing that really works is modest and old: time, water, rest, eating something, and giving your body the room to do its work. And the only real prevention is moderation beforehand.¹²

A thought, without a wagging finger

I am not writing this to forbid you anything. A glass of wine over a good conversation, a toast to life, that is part of being human, and there is nothing wrong with it. This article is not about morality. It is about understanding.

But there is a gentle thought within all this science. When you understand exactly what is happening, the alcohol your liver has to clear, the inflammation, the disturbed sleep, the body that can carry it for days, then perhaps you look at it just a little more consciously. Not with guilt, but with knowledge.

Because that, in the end, is what matters to me: not rules, but awareness. When you know what something does to your body, you can make a freer, more considered choice. Sometimes that is one glass fewer. Sometimes skipping an evening. Sometimes simply: enjoying, and carrying the morning after with kindness towards yourself, knowing what is happening inside.

In closing

The hangover is, like so much in our body, a sign. A sign that you have limits, that your body responds, that everything you take in has to be processed somewhere. It is not an enemy to fight, but a messenger to listen to.

How do you listen to your body’s signals, not only after an evening of drinking, but in general? Do you notice when something does you good and when something drains you, and do you let that knowledge guide your choices?

No quick answer. Just a question to carry with you, the next time your body tries to tell you something.

Written with care, this article describes the science and is not medical advice. If you have concerns about your health or your alcohol use, please discuss them with your GP.

Related reading

• Why Sleeping In Gives You a Headache
• The liver: the organ the ancients revered
• Glutamine: Your Gut’s Favourite Food

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Sources

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Sources on this site are cited in APA style, with a DOI wherever one exists. Why I made that choice →