We have all been there. You wake up with a pounding headache, a churning stomach, and a sincere promise to yourself that you will never drink again. But what is actually going on inside your body when you are hungover? Understanding the science can help you make better choices — both about how much you drink and how you recover.
Let’s break down what happens from the moment alcohol enters your system to the miserable morning after.
How Your Body Processes Alcohol
Before we can understand hangovers, we need to understand how your liver handles alcohol. When you have a drink, your body treats ethanol (the active ingredient in alcoholic beverages) as a toxin and prioritises breaking it down.
The Two-Step Breakdown
Your liver metabolises alcohol in two main stages:
-
Ethanol to acetaldehyde. An enzyme called alcohol dehydrogenase (ADH) converts ethanol into acetaldehyde — a highly toxic compound that is between 10 and 30 times more toxic than alcohol itself.
-
Acetaldehyde to acetate. A second enzyme, aldehyde dehydrogenase (ALDH2), then converts acetaldehyde into acetate, a relatively harmless substance your body can use for energy or excrete.
The problem is that your liver can only process roughly one standard drink per hour. When you drink faster than that, acetaldehyde builds up in your bloodstream, and that is when the trouble really starts.
Why Acetaldehyde Is the Main Villain
Acetaldehyde is responsible for many of the classic hangover symptoms. It triggers inflammation throughout your body, damages cell membranes, and irritates the lining of your stomach and intestines. Research published in Alcohol and Alcoholism has confirmed that elevated acetaldehyde levels correlate directly with hangover severity.
Think of it this way: your liver is running a detoxification assembly line, and when you overload it, the half-processed toxic waste backs up into the rest of your system.
Dehydration: More Than Just Being Thirsty
One of the most well-known effects of alcohol is dehydration, but the mechanism is more nuanced than most people realise.
How Alcohol Dehydrates You
Alcohol suppresses the release of vasopressin (also called antidiuretic hormone or ADH — not to be confused with the liver enzyme). Vasopressin normally tells your kidneys to reabsorb water. Without it, your kidneys send that water straight to your bladder instead.
This is why you need to visit the bathroom so frequently when you are drinking. For every standard drink you consume, your body can expel up to 120 millilitres more urine than the volume of the drink itself. Over a big night, that adds up to a significant fluid deficit.
The Consequences of Dehydration
Dehydration contributes to several hangover symptoms:
- Headaches. Your brain is about 75 per cent water. When you are dehydrated, your brain can temporarily shrink and pull away from the skull, triggering pain receptors in the surrounding membrane. This is the mechanical basis for that splitting hangover headache.
- Dry mouth and thirst. Your salivary glands need adequate hydration to function, and your body’s thirst signals go into overdrive.
- Fatigue and dizziness. Reduced blood volume means less oxygen delivery to your muscles and brain.
- Electrolyte imbalance. Along with water, you lose essential minerals like sodium, potassium, and magnesium — all of which are crucial for normal muscle and nerve function.
The Inflammatory Response
A hangover is, in many ways, an inflammatory event. Research from Kyungpook National University in South Korea found that hangover severity correlates strongly with elevated levels of inflammatory markers in the blood, including cytokines like interleukin-10, interleukin-12, and interferon-gamma.
What Triggers the Inflammation
Several factors drive the inflammatory cascade:
- Acetaldehyde damage. As discussed, this toxic metabolite directly damages tissues and triggers your immune system to respond.
- Gut permeability. Alcohol increases the permeability of your intestinal lining (sometimes called “leaky gut”), allowing bacterial endotoxins to enter your bloodstream. Your immune system reacts to these endotoxins with a full inflammatory response.
- Oxidative stress. Alcohol metabolism generates free radicals — unstable molecules that damage cells and DNA. Your body’s antioxidant defences become overwhelmed, leading to oxidative stress.
Why You Feel Like You Have the Flu
The inflammatory cytokines released during a hangover are the same molecules your body produces when you are fighting an infection. That is why a bad hangover can feel remarkably similar to having the flu: body aches, fatigue, brain fog, nausea, and a general sense of malaise are all driven by your immune system’s inflammatory response rather than the alcohol itself.
Your Immune System Takes a Hit
Beyond the inflammatory response, alcohol directly suppresses your immune system. Research has shown that even a single episode of heavy drinking can reduce the activity of natural killer cells and alter cytokine production for up to 24 hours afterwards.
This immune suppression is one reason why you might feel run-down or catch a cold in the days following a big night out. Your body is simultaneously trying to recover from the toxic effects of alcohol while operating with compromised defences.
Electrolyte Imbalance: The Hidden Problem
While dehydration gets most of the attention, electrolyte imbalance is arguably just as important to how awful you feel.
Key Electrolytes Affected
- Potassium. Essential for heart rhythm, muscle contraction, and nerve signalling. Low potassium contributes to muscle weakness, cramps, and fatigue.
- Magnesium. Involved in over 300 enzymatic reactions in your body. Depletion leads to headaches, muscle tension, irritability, and poor sleep quality.
- Sodium. Critical for fluid balance and nerve function. Loss contributes to dizziness, confusion, and nausea.
- Zinc. Important for immune function and enzyme activity. Even moderate drinking depletes zinc stores.
Replenishing these electrolytes — not just water — is a key part of hangover recovery, which is why plain water alone often does not make you feel much better.
Congeners: Why Some Drinks Hit Harder
Not all alcoholic drinks produce equally bad hangovers, and the reason comes down to congeners.
What Are Congeners?
Congeners are biologically active compounds produced during the fermentation and ageing process. They include substances like methanol, tannins, fusel oils, and various aldehydes. While they contribute to the flavour, aroma, and colour of alcoholic beverages, they also add to the toxic burden your body must process.
High-Congener vs Low-Congener Drinks
Research published in Current Drug Abuse Reviews confirmed that high-congener drinks produce significantly worse hangovers than low-congener alternatives at the same blood alcohol level.
High-congener drinks (worse hangovers):
- Bourbon and whiskey
- Red wine
- Brandy and cognac
- Dark rum
- Tequila (especially gold varieties)
Low-congener drinks (comparatively milder hangovers):
- Vodka
- White wine
- Gin
- Light rum
- Light-coloured beer
This does not mean low-congener drinks are “safe” — they still cause hangovers through all the other mechanisms described here. But if you are comparing like for like, a night on bourbon will generally leave you feeling worse than a night on vodka.
Sleep Disruption: Why You Wake Up Exhausted
You might think alcohol helps you sleep — after all, it can make you feel drowsy. But alcohol is actually one of the worst things for sleep quality.
How Alcohol Wrecks Your Sleep
- REM suppression. Alcohol significantly reduces the amount of REM (rapid eye movement) sleep you get. REM sleep is critical for memory consolidation, emotional regulation, and feeling rested.
- Sleep fragmentation. As your blood alcohol level drops during the night, you experience a “rebound” effect that causes frequent awakenings, often in the second half of the night.
- Breathing disruption. Alcohol relaxes the muscles in your throat, which can worsen snoring and sleep apnoea, further reducing sleep quality.
- Early waking. Many drinkers find themselves wide awake at 4 or 5 am, unable to get back to sleep. This is partly due to the rebound effect and partly due to drops in blood sugar.
Even if you were in bed for eight hours, the quality of sleep you got after drinking heavily might be equivalent to only four or five hours of proper rest. This sleep debt compounds the fatigue, brain fog, and irritability of the hangover itself.
Stomach Irritation and Nausea
That queasy, unsettled stomach is not just in your head — alcohol directly irritates the lining of your gastrointestinal tract.
The Mechanisms
- Increased acid production. Alcohol stimulates your stomach to produce more hydrochloric acid, which can irritate the stomach lining and contribute to nausea and reflux.
- Delayed gastric emptying. Alcohol slows the movement of food through your digestive system, leading to that heavy, bloated feeling.
- Mucosal damage. Concentrated alcohol can physically damage the protective mucous layer of your stomach, leading to inflammation (gastritis).
- Gut motility changes. Alcohol can cause both constipation and diarrhoea by disrupting normal gut motility patterns.
Why Some People Get Worse Hangovers
If you have ever wondered why your mate can drink the same amount as you and bounce out of bed the next morning while you are incapacitated, genetics plays a significant role.
The Genetics of Alcohol Metabolism
The efficiency of your ADH and ALDH2 enzymes is largely determined by your genes.
- ALDH2 deficiency. Approximately 36 per cent of East Asian populations carry a variant of the ALDH2 gene (called ALDH2*2) that makes the enzyme significantly less effective. People with this variant accumulate acetaldehyde much more rapidly, leading to facial flushing, nausea, and dramatically worse hangovers. This is commonly known as “Asian flush” or “alcohol flush reaction.”
- ADH variants. Some people have more active forms of ADH, meaning they convert ethanol to acetaldehyde faster than their ALDH2 can clear it. This creates a toxic bottleneck.
- Cytokine response. Genetic variation in inflammatory cytokine production means some people mount a much stronger immune response to the same amount of alcohol.
Other Factors That Influence Hangover Severity
Beyond genetics, several other variables matter:
- Age. Hangovers tend to worsen as you get older. Liver enzyme efficiency declines, and your body becomes less resilient to oxidative stress and dehydration.
- Sex. Women tend to experience worse hangovers than men at the same consumption level, partly because women generally have less body water (and therefore achieve higher blood alcohol concentrations) and produce less ADH in the stomach lining.
- Body composition. Higher body fat percentage means less water to dilute alcohol, leading to higher blood alcohol levels.
- Hydration status. If you were already dehydrated before drinking (common in the Australian climate), your hangover will likely be worse.
- Food intake. Drinking on an empty stomach leads to faster alcohol absorption and worse outcomes.
- Medications. Certain medications, including common over-the-counter painkillers, can interact with alcohol and worsen hangover symptoms.
- Sleep quality. If your sleep was already poor before the drinking session, the compounding effect will be significant.
A Note on “Hangxiety”
Many people experience significant anxiety during a hangover — a phenomenon sometimes called “hangxiety.” This is not imaginary. Alcohol initially boosts the activity of GABA (your brain’s primary calming neurotransmitter) and suppresses glutamate (an excitatory neurotransmitter). When the alcohol wears off, your brain rebounds in the opposite direction: GABA activity drops and glutamate surges, creating a neurochemical state that is primed for anxiety, restlessness, and irritability.
If you already experience anxiety, hangovers can be particularly distressing. This is worth keeping in mind when making decisions about how much to drink.
The Bottom Line
A hangover is not a single event but a complex, multi-system response involving:
- Toxic acetaldehyde accumulation
- Dehydration and electrolyte depletion
- Systemic inflammation and immune activation
- Sleep disruption and neurotransmitter imbalance
- Gastrointestinal irritation
- Oxidative stress
Understanding these mechanisms can help you make informed decisions about drinking and recovery. The most effective way to prevent a hangover is, of course, to drink less or not at all. But when you do choose to drink, pacing yourself, staying hydrated, eating beforehand, and choosing lower-congener drinks can all reduce the severity of what follows.
If you or someone you know is struggling with alcohol use, the National Alcohol and Other Drug Hotline is available 24 hours a day on 1800 250 015. It is free, confidential, and staffed by trained counsellors.
Health Disclaimer: This article is for informational purposes only and does not constitute medical advice. The content is based on published scientific research but should not be used as a substitute for professional medical guidance. If you have concerns about your health or alcohol consumption, please consult a qualified healthcare professional. If you are experiencing a medical emergency, call 000 immediately.