What Causes a Hangover? The Science Explained

Let's get one thing straight: a hangover isn't just dehydration. That's the TikTok version, the oversimplified Instagram carousel explanation. The actual reason you're face-down in bed at 2 PM on a Saturday, questioning every life choice, involves a toxic metabolite, your immune system going into full panic mode, and your nervous system basically bouncing off the walls.

Your body isn't punishing you. It's simply confused by what just happened. And once you understand the why, you can actually do something smarter about the how when it comes to recovery.

Let's break down the actual science behind what causes hangovers.

It Starts With Acetaldehyde (And It's Toxic)

Here's the uncomfortable truth: alcohol isn't the villain — it's what your body does with alcohol that matters.

When you drink, your liver goes to work immediately. It converts ethanol (the actual alcohol) into acetaldehyde through an enzyme called alcohol dehydrogenase. Then another enzyme, aldehyde dehydrogenase (ALDH2), should convert that acetaldehyde into acetate, which your body can handle. (This is well-established alcohol-metabolism science.)

The problem? Acetaldehyde is legitimately toxic. We're talking nausea, flushing, headaches, and general misery. And the speed at which your liver can clear acetaldehyde depends largely on your ALDH2 enzyme activity, which is partly genetic.

This is why some people feel "fine" the morning after, while others are absolutely wrecked. If you've got lower ALDH2 activity (which is common in East Asian populations — affecting roughly a third — due to a genetic variant), acetaldehyde hangs around longer, and you feel worse.

You know that "Asian glow" people talk about? Red face, nausea, feeling ill after just one or two drinks? It's essentially alcohol-induced flushing and a sign of lower ALDH2 activity. Your liver is struggling to clear acetaldehyde efficiently, and the buildup is responsible for a disproportionately severe response.

Before we even talk about dehydration, inflammation, or sleep disruption, the first domino to fall is acetaldehyde accumulation. It's literally a poison your body is trying to expel.

Dehydration is Part of it (But Not the Whole Story)

Yes, alcohol is a diuretic, and you're losing water. No, drinking a litre of water before bed won't fix a hangover (we've all tried — it doesn't work).

What's actually happening is alcohol suppresses antidiuretic hormone (also called vasopressin) — confusingly abbreviated "ADH" in some textbooks, but a completely different molecule from the alcohol dehydrogenase enzyme above. This hormone normally tells your kidneys to reabsorb water and reduce urine output. When alcohol suppresses its release, your kidneys just... keep making urine.

You're also losing electrolytes: sodium, potassium, and magnesium. These aren't just random minerals — they regulate nerve function, muscle contraction, and hydration at the cellular level. Losing these electrolytes without replacing them is why water alone feels useless, as your cells can't hold onto it.

Dehydration contributes to the pounding headache and that gross dry-mouth feeling, but it's not the whole picture. People who've had a full glass of water between every drink and still feel dreadful the next day know this instinctively. There's more going on.

That's where the immune system comes in.

Your Immune System Goes Into Overdrive

This is arguably the biggest culprit for why you feel ill during a hangover — like you've caught the flu.

Alcohol is a toxin. Your body recognises it as such, and research suggests your immune system responds accordingly. Studies have linked hangover severity to a rise in inflammatory molecules called cytokines — with different studies reporting increases in markers such as IL-6, IL-10, IL-12 and TNF-α (the exact pattern varies between studies). The picture that emerges is your body treating the aftermath like a low-grade inflammatory event.

This cytokine response is thought to be part of why the morning after feels like being ill:

• Muscle aches and general body pain
• Fatigue and brain fog
• Nausea and loss of appetite
• That bone-deep tiredness that sleep doesn't fix

Paracetamol might help with the headache, but it doesn't address the inflammation itself. Your body is inflamed, which takes time to settle.

The severity of this response appears to vary based on factors like:

• How much you drank
• What you drank
• Whether you ate
• Your individual physiology

This inflammatory side of things also helps explain why you can't just "push through". Your body is working to mount and then wind down a response, and rest genuinely helps it do that.

The Glutamate Rebound: Why You Can't Sleep

Alcohol is a central nervous system depressant. It muffles your brain. So logically, you'd expect to sleep brilliantly after drinking. You probably did sleep — but it was terrible quality, and you woke up at 3 AM for no reason.

Why? Alcohol dampens glutamate, an excitatory neurotransmitter, while boosting calming GABA signalling. Your brain adapts by ramping glutamate activity back up. When you stop drinking and the alcohol leaves your system, you can get a rebound in this excitatory signalling — which helps explain insomnia, vivid dreams, night sweats, and early-morning waking.

This is the glutamate rebound effect, and it's a big reason why "just getting more sleep" doesn't feel restorative. Your sleep was fragmented, your REM cycles were disrupted, and your nervous system spent the night in a partially activated state.

This same rebound excitability is also thought to fuel that "hangxiety" — the weirdly intense, wired feeling you get the morning after. Your brain is overstimulated despite the fact that you feel physically destroyed. It's a maddening combination — exhausted but unable to rest.

Congeners: Not All Drinks Are Equal

You've probably noticed that some drinks hit differently. Red wine responses feel different from vodka responses. There's actual science here.

Congeners are compounds produced during fermentation and distillation — things like acetone and methanol. (Wine also contains tannins and sulfites, which aren't congeners in the strict sense but can add to the effect.) Congeners are especially high in dark spirits and wines.

Dark rum, bourbon, and red wine have significantly higher congener content than clear spirits like vodka and gin. In a controlled trial comparing bourbon with vodka, people reported worse hangovers after the high-congener bourbon at the same alcohol dose. Worth noting, though: that same research found the amount of alcohol matters far more than the type.

Methanol is the one researchers tend to single out. In small amounts (naturally present in all fermented drinks), your body converts it to formaldehyde and formic acid — both unpleasant — and some studies suggest this may contribute to how rough you feel. Fortified wines and low-quality spirits can carry more methanol.

Wine also contains sulfites (added as a preservative), and some people are sensitive to these independently of the alcohol effect. They can trigger headaches and nausea on their own.

So the type of alcohol may affect your hangover — but it's a smaller lever than how much you drink. If you're prone to severe responses, sticking to clear spirits and light beers (and drinking less overall) can reduce the load.

Your Gut is Having a Bad Time Too

While your liver processes acetaldehyde and your immune system flares up, your gut lining is getting genuinely irritated.

Alcohol irritates and can damage the protective lining of your stomach and intestines (its effect on stomach-acid production depends on the drink — lower-alcohol drinks like beer and wine tend to stimulate it, while strong spirits don't). The result can be nausea, loss of appetite, and sometimes actual stomach pain. This isn't imaginary — your gut lining is inflamed.

Even a single heavy session can measurably raise gut-derived endotoxin in the blood — a marker of increased intestinal permeability (the "leaky gut" thing you hear about). With regular heavy drinking, alcohol can also shift the gut microbiome and disrupt the gut barrier, feeding into the overall inflammatory state and the poor digestion that comes with a hangover.

This is also why eating greasy food the next day can feel impossible — your stomach is inflamed and your digestive system is offline. Bland carbs and electrolytes make way more sense.

Why Some People Get Worse Responses

Not everyone's response is created equal. A few variables stack the deck:

Genetics

As mentioned, ALDH2 enzyme activity is partially inherited. If you have variants that slow acetaldehyde clearance, you're fighting an uphill battle. There's little you can do about this — it's baked into your genes.

Age

Here's a surprise that runs against the popular wisdom: large surveys suggest hangovers actually become less frequent and less severe with age, not worse. One major study of over 50,000 adults found younger drinkers were far more likely to report severe hangovers than the over-60s — and that wasn't fully explained by how much they drank. Other research points the same way, with the link between age and hangover severity strongly tied to how intoxicated people felt in the first place. If your own hangovers feel worse than they used to, the more likely culprits are changes in your drinking patterns, sleep, hydration, or recovery time — not your age alone.

Hydration Status Before Drinking

If you start the night dehydrated, the diuretic effect of alcohol hits harder. Starting well hydrated doesn't eliminate the response, but it can reduce its severity.

Food

Eating before and while drinking slows alcohol absorption, keeps your blood sugar stable, and provides nutrients your body needs to handle the metabolic stress. Empty-stomach drinking is a unilateral bad decision.

Sleep Quality

Poor sleep before drinking can worsen how you feel. Your body's already running a deficit of rest capacity, and alcohol's sleep-disrupting effects pile on top. Start the night tired, and you'll likely feel worse the next morning.

How Much You Drank

The more alcohol you drink, the worse you'll likely feel. Drinking faster also means higher peak blood alcohol levels, which stresses your liver more and is associated with a more intense response.

Frequently Asked Questions: Hangover Causes

So What Now?

You now understand the actual mechanisms: acetaldehyde toxicity, immune-system inflammation, dehydration at the electrolyte level, nervous-system rebound, congener load, and gut irritation. It's not one thing. It's a cascade.

This is why a hangover is so hard to recover from — it's multifactorial. You're not just thirsty. You're dealing with a toxic metabolite, an inflammatory response, electrolyte loss, nervous-system chaos, sleep disruption, and a compromised gut. Water and paracetamol touch only part of the problem.

Smart recovery means supporting your body across the board: replacing electrolytes, replenishing B vitamins (which regular drinking can deplete — especially thiamine), supporting normal nervous-system function with magnesium, and getting antioxidant nutrients like vitamin C and zinc that help protect cells from oxidative stress.

You've got sh*t to do. Support your body properly on the back end, and the next day doesn't have to be a complete write-off.