What Causes a Hangover? The Real Science Behind the Morning After

By NUDAY Editorial · Reviewed by NUDAY Research Team · Last updated May 2026

"What causes a hangover" is one of the most-asked health questions on the internet — and the most poorly answered. Most articles list two or three causes (dehydration, alcohol, "toxins") and call it done.

The honest answer is that nine separate biological mechanisms drive hangover symptoms, and they all peak at roughly the same time. Understanding each one is what separates effective recovery from the "drink water and hope" approach. This guide breaks down the science of every cause, what each one is responsible for, and how proper recovery actually works. Every claim is sourced to published research.

What is a hangover, biologically?

A hangover is the body's recovery response to alcohol — a cluster of physiological symptoms that begin once alcohol has been mostly cleared from the bloodstream and the body is repairing the damage alcohol caused along the way.

Crucially, a hangover is not caused by the presence of alcohol — it's caused by what happens after alcohol is gone. Symptoms typically peak when blood alcohol concentration (BAC) returns to zero, roughly 6–8 hours after drinking stops. This is when the body has fully metabolised the alcohol into acetaldehyde and then into acetate, when dehydration is maximal, and when sleep disruption from the night before is hitting hardest.

According to a 2012 review in Current Drug Abuse Reviews, the pathology of a hangover involves "a number of biochemical, physiological, and pathological processes" working in parallel. The reason the symptoms feel coordinated is that all these processes share the same timeline.

What are the main causes of a hangover?

The nine main causes of a hangover are: acetaldehyde toxicity, dehydration, electrolyte depletion, sleep disruption, inflammation, blood sugar drops, vitamin depletion, gut irritation, and congener exposure — each contributing different symptoms that compound together.

Here's a quick map of which cause produces which symptom:

Cause	Primary symptoms
Acetaldehyde toxicity	Headache, nausea, sweating, rapid heartbeat
Dehydration	Thirst, dry mouth, dizziness, fatigue
Electrolyte depletion	Muscle cramps, weakness, irregular heartbeat
Sleep disruption	Exhaustion, brain fog, irritability
Inflammation	Body aches, headache, cognitive slowing
Blood sugar drop	Shakiness, sugar cravings, mood swings
Vitamin depletion	Fatigue, brain fog, low mood
Gut irritation	Nausea, stomach pain, diarrhoea
Congeners	Multiplies severity of all the above

This is why a hangover feels like multiple problems happening at once — because it is multiple problems happening at once. Now let's break down each cause.

Cause 1 — Acetaldehyde toxicity

Acetaldehyde is a highly toxic compound the body produces when it breaks down alcohol — it's 10 to 30 times more toxic than alcohol itself and is the single biggest driver of classic hangover symptoms.

Here's the metabolic chain: when you drink, your liver breaks alcohol (ethanol) into acetaldehyde using the enzyme alcohol dehydrogenase. Acetaldehyde then needs to be further broken down into acetate (a harmless compound) using a second enzyme called aldehyde dehydrogenase (ALDH2). The acetaldehyde stage is where the damage happens.

Acetaldehyde causes:

• Headache and migraine-like pain
• Nausea and vomiting
• Facial flushing (especially in people with ALDH2 variants)
• Rapid heartbeat
• Sweating
• Inflammation across multiple tissues

The body clears acetaldehyde at a relatively fixed rate, regardless of how much you drank. Drink more than the liver can process, and acetaldehyde accumulates — producing worse and longer hangovers.

This is why ingredients that support acetaldehyde clearance matter. NUDAY Next-Day Support Gummies contain two such ingredients: 500mg DHM (which has been studied for its effects on alcohol metabolism enzymes) and L-cysteine (an amino acid involved in acetaldehyde detoxification). A 2020 placebo-controlled study published in Alcohol and Alcoholism showed L-cysteine supplementation alongside drinking significantly reduced hangover symptoms including nausea and headache.

Cause 2 — Dehydration

Alcohol causes dehydration because it suppresses vasopressin — the hormone that tells your kidneys to retain water — leading to roughly 4 times more urine output per unit of alcohol consumed.

The mechanism is well-documented. Normal vasopressin levels signal the kidneys to reabsorb water back into the bloodstream. Alcohol blocks this signal, so water that should be retained gets excreted instead. According to a 2017 review in the European Journal of Clinical Nutrition, every standard drink causes the body to lose approximately 100ml more fluid than the drink itself contained.

Over a night of drinking — say four standard drinks — that's an additional 400ml of fluid lost on top of the alcohol's own dehydrating effect. By morning, total fluid deficit can easily reach 1.5–2 litres.

What dehydration causes:

• Thirst, dry mouth, dry skin
• Dizziness when standing up
• Reduced blood volume → fatigue and brain fog
• Headaches as brain tissue temporarily shrinks
• Concentrated urine and reduced output

The trap most people fall into: drinking plain water before bed and assuming that solves it. Plain water restores volume but not the electrolytes alcohol caused you to lose alongside the water. This brings us to the next cause.

Cause 3 — Electrolyte depletion

Alcohol causes the body to lose sodium, potassium, magnesium, and chloride alongside the water — and water alone cannot restore them, which is why people who diligently drink water before bed still wake up with hangover symptoms.

Every litre of urine you produce while drinking carries roughly:

• 1,500–3,000mg sodium
• 1,500–3,000mg potassium
• Trace amounts of magnesium and calcium

This is why hydration is fundamentally a mineral problem, not just a water problem. Water without electrolytes passes through your system without rehydrating cells. According to the 2017 ACSM position stand on fluid replacement, sodium is the critical mineral for active rehydration — and standard hangover hydration advice ignores it almost entirely.

Electrolyte depletion contributes to:

• Muscle cramps, particularly calves and feet
• Weakness and fatigue beyond what dehydration alone produces
• Headaches that don't respond to water
• Heart palpitations or irregular rhythm
• Brain fog

NUDAY Next-Day Support Gummies include electrolyte support directly in the formula — sodium, potassium, and magnesium combined with DHM and L-cysteine — so the mineral side of recovery is handled alongside the metabolic side.

Cause 4 — Sleep disruption

Alcohol disrupts sleep architecture by reducing REM sleep (the restorative phase) by ~25% and fragmenting deep sleep stages, meaning even 8 hours of sleep after drinking produces only the recovery effect of 5–6 hours of normal sleep.

The pattern is consistent across decades of sleep research. Alcohol initially produces drowsiness and sleep onset — which is why people use it to fall asleep — but the second half of the night becomes fragmented. REM sleep is suppressed for the first half of the night, then "rebounds" in the second half, producing vivid or disturbing dreams. Total sleep efficiency drops by 9–35% depending on dose.

This sleep disruption contributes to:

• Morning exhaustion that doesn't match how long you "slept"
• Brain fog and reduced cognitive performance
• Mood disturbance, irritability, "hangxiety"
• Slowed reaction time and decision-making
• Reduced motor coordination

The implication: no supplement can fully fix the cognitive side of a hangover until your sleep deficit has been recovered. This usually requires 1–2 nights of proper sleep after the drinking night.

Cause 5 — Inflammation

Alcohol triggers a systemic inflammatory response — releasing cytokines and inflammatory markers that cause muscle aches, brain fog, and the "feels like the flu" sensation of severe hangovers.

The inflammation is partly a direct response to alcohol's toxicity and partly a response to acetaldehyde damage to tissues. A 2016 study in Alcoholism: Clinical and Experimental Research measured cytokine levels in people with hangovers and found significantly elevated levels of inflammatory markers like IL-6, IL-10, and TNF-alpha.

Inflammation contributes to:

• Body aches and joint pain
• Headache (alongside dehydration's contribution)
• Reduced mental clarity and processing speed
• The "rundown" feeling that's hard to localise
• Increased perceived stress and emotional reactivity

This is also why anti-inflammatory foods, sleep, and time matter more than any single supplement. The inflammation has to resolve on its own timeline.

Cause 6 — Blood sugar drops

Alcohol disrupts the liver's ability to produce glucose (gluconeogenesis), which can cause hypoglycaemia (low blood sugar) — particularly in the morning when blood sugar would normally be replenished.

Normally when blood sugar drops, the liver releases stored glucose back into the bloodstream. Alcohol blocks this process while it's being metabolised. The result is that blood sugar can drop below normal levels — particularly if you drank on an empty stomach or skipped food during drinking.

What blood sugar drops cause:

• Shakiness and weakness
• Intense sugar and carb cravings
• Mood swings and irritability
• Cognitive slowing
• Sweating and lightheadedness

This is why the classic post-drinking food cravings tend toward simple carbs and sugar — your body is trying to restore blood glucose. The smarter approach is balanced food with protein and complex carbs, which restores blood sugar more sustainably.

Cause 7 — Vitamin depletion (especially B-vitamins)

Alcohol depletes B-vitamins — particularly B1 (thiamine), B6, and B12 — because alcohol metabolism consumes these vitamins as cofactors and because alcohol impairs absorption of new B-vitamins from food.

The most affected vitamins:

• B1 (thiamine): Critical for energy production and nervous system function. Severe deficiency causes neurological symptoms.
• B6 (pyridoxine): Involved in neurotransmitter production and immune function.
• B12 (cobalamin): Required for nerve function and red blood cell production.
• Folate (B9): Important for cellular repair and methylation.
• Vitamin C: Antioxidant depleted during inflammatory response.

B-vitamin depletion contributes to morning fatigue, brain fog, and slower recovery. This is why proper hangover formulations include a B-complex — replenishing what alcohol consumed. NUDAY Next-Day Support Gummies include a B-complex specifically to address this depletion.

Cause 8 — Gut irritation

Alcohol directly irritates the stomach lining, increases gastric acid production, slows gastric emptying, and disrupts gut microbiome balance — producing the nausea, stomach pain, and digestive issues common in hangovers.

The gut effects of alcohol are immediate and persistent. Even small amounts increase stomach acid; larger amounts can cause inflammation of the stomach lining (gastritis) that takes 24–48 hours to resolve. According to a 2017 review in Alcohol Research: Current Reviews, alcohol also disrupts the gut microbiome, reducing beneficial bacteria and allowing harmful bacteria to proliferate.

Gut irritation drives:

• Nausea and vomiting
• Stomach pain and burning
• Loss of appetite
• Diarrhoea
• Bloating and discomfort

This is why heavy or fatty food often makes hangovers worse rather than better — an already-irritated stomach struggles to process rich food. Light, balanced meals work better.

Cause 9 — Congeners (the dark spirit problem)

Congeners are chemical compounds produced during fermentation and aging of alcoholic drinks — and they significantly worsen hangover severity, with dark spirits containing 5–10 times more congeners than clear spirits.

The congener content roughly tracks with colour: clear vodka contains the lowest amounts; gin and white wine somewhat higher; whiskey, bourbon, brandy, and red wine contain the highest concentrations.

A 2010 study in Alcoholism: Clinical and Experimental Research compared bourbon and vodka hangovers at matched alcohol doses. The bourbon group reported significantly worse hangover symptoms across nausea, headache, and fatigue, despite identical BAC levels.

Common congeners include:

• Methanol (especially in distilled spirits and red wine)
• Acetone
• Various fusel alcohols (propanol, butanol, isobutanol)
• Furfural
• Tannins (in red wine and dark spirits)

The practical implication: if you want to minimise hangover severity at a given alcohol intake, lighter and clearer drinks produce milder hangovers than dark spirits and red wine.

Why do some people have worse hangovers than others?

Individual hangover severity varies dramatically based on genetics, body composition, age, baseline health, hydration status, and whether alcohol was consumed alongside food.

ALDH2 genetic variants

Roughly 36% of East Asian populations carry a variant of the ALDH2 gene that produces a less effective version of the acetaldehyde-clearing enzyme. People with this variant experience faster acetaldehyde accumulation, more intense facial flushing, and significantly worse hangovers — even from small amounts of alcohol.

Body composition

The same amount of alcohol produces higher BAC in smaller bodies and bodies with lower water content. This is why women generally experience higher BAC than men at the same dose, and why older people experience higher BAC than younger people at the same dose.

Age

Hangovers worsen progressively with age due to declining body water percentage, slower liver enzymes, worse sleep architecture, and reduced inflammatory tolerance. Same drinking pattern at 22 vs 42 produces very different recovery times.

Hydration baseline

People who start the drinking session already dehydrated experience worse hangovers than people who started well-hydrated.

Food

Drinking on an empty stomach causes alcohol to absorb faster, pushing BAC higher and producing worse hangovers. Eating before and during drinking — particularly meals with protein and fat — slows absorption.

How NUDAY Next-Day Support targets the major causes

NUDAY Next-Day Support Gummies are formulated to address the four largest hangover mechanisms — acetaldehyde toxicity, dehydration, electrolyte depletion, and vitamin depletion — in a single Stage-1 (pre-sleep) formula.

Hangover cause	NUDAY ingredient	Dose
Acetaldehyde toxicity	DHM (Dihydromyricetin)	500mg (3–10x market avg)
Acetaldehyde clearance	L-Cysteine	Included
Vitamin depletion	B-Complex	Included
Electrolyte depletion	Electrolyte support	Included
Serving	2 gummies before bed	—

NUDAY Next-Day Support Gummies are designed to be taken pre-sleep, before symptoms develop, so the active ingredients are supporting your body's natural overnight recovery — not arriving once damage is already done.

For the morning-after protocol once symptoms have already developed, see our companion guides: How to Cure a Hangover and How Long Does a Hangover Last.

Frequently Asked Questions

Is dehydration the main cause of a hangover?

Dehydration is one of nine causes — and not even the biggest one. Acetaldehyde toxicity, the byproduct of alcohol metabolism, is typically considered the largest single driver of hangover symptoms. This is why drinking water alone rarely prevents hangovers; it addresses only one of nine mechanisms.

What is the main toxic substance in a hangover?

Acetaldehyde — produced when the liver breaks down alcohol. Acetaldehyde is 10 to 30 times more toxic than alcohol itself and is responsible for most of the classic hangover symptoms: headache, nausea, sweating, rapid heartbeat. The body clears it through a second enzyme (ALDH2), but the clearance rate is fixed regardless of dose.

Why do I get hungover after only 1–2 drinks?

Several reasons: ALDH2 genetic variants (slower acetaldehyde clearance), lower body water percentage, drinking on an empty stomach, starting already dehydrated, alcohol intolerance, or specific drink choices (dark spirits, red wine, sugary cocktails). Some people are genuinely more sensitive due to physiology, not amount.

Does the type of alcohol affect hangover severity?

Yes — significantly. Dark spirits (whiskey, bourbon, brandy, red wine) contain 5–10 times more congeners than clear spirits (vodka, gin). Congeners are fermentation byproducts that worsen hangover symptoms independently of alcohol content. Clear drinks generally produce milder hangovers.

Why does my hangover include anxiety (hangxiety)?

Alcohol disrupts the balance between GABA (calming neurotransmitter) and glutamate (excitatory neurotransmitter). When alcohol leaves the system, there's a temporary glutamate overshoot — producing anxiety, racing thoughts, and emotional reactivity. Combined with sleep deprivation and blood sugar drops, this creates the "hangxiety" phenomenon.

Can sleep prevent a hangover?

Good sleep helps but cannot prevent a hangover on its own. Alcohol disrupts sleep architecture regardless of how long you sleep, so even 10 hours of sleep after heavy drinking doesn't produce the recovery of 8 hours of normal sleep. The most effective approach is reducing alcohol intake, eating alongside drinking, hydrating with electrolytes, and using ingredients that target acetaldehyde clearance.

Why do hangovers get worse with age?

Multiple biological changes: body water percentage drops (less dilution of alcohol), liver enzymes slow (acetaldehyde clears more slowly), sleep architecture deteriorates (less REM, more fragmentation), kidney function declines (slower rehydration), and inflammatory tolerance decreases. Same drinking pattern produces longer and more severe hangovers progressively with age.

Is alcohol allergic reaction the same as a hangover?

No. True alcohol allergy or intolerance produces immediate symptoms — facial flushing, hives, nasal congestion, rapid heart rate — within minutes to hours of drinking. A hangover is a delayed recovery response that peaks after BAC returns to zero. The two can co-occur but have different mechanisms.

Can you "build tolerance" to hangovers?

Not really. You can build tolerance to alcohol's intoxication effects, meaning you feel less drunk at higher BAC — but this doesn't reduce the acetaldehyde production, dehydration, or other hangover mechanisms. People with high alcohol tolerance often experience worse hangovers because they drink more without feeling intoxicated.

Why does coffee make my hangover worse sometimes?

Caffeine is a diuretic, adding to existing dehydration. It also temporarily masks fatigue without resolving sleep debt, can irritate an already-inflamed stomach, and increases anxiety on top of hangxiety. Coffee isn't bad alongside proper rehydration but isn't a hangover remedy on its own.

Key Takeaways

• A hangover is caused by nine separate biological mechanisms, not just dehydration
• Acetaldehyde toxicity — the byproduct of alcohol metabolism — is the largest single driver of hangover symptoms
• Alcohol causes ~4x more fluid loss than its volume, plus electrolyte depletion that water alone cannot fix
• Even moderate alcohol consumption reduces REM sleep by ~25%, producing morning exhaustion regardless of total sleep time
• Inflammation, blood sugar drops, vitamin depletion, gut irritation, and congeners all contribute additional symptoms
• Dark spirits and red wine contain 5–10x more congeners than clear spirits, producing significantly worse hangovers
• Individual hangover severity varies based on genetics (ALDH2), body composition, age, food intake, and hydration baseline
• Effective recovery requires targeting multiple causes simultaneously — not just drinking water
• NUDAY Next-Day Support Gummies are formulated to address the four largest causes — acetaldehyde toxicity, dehydration, electrolyte depletion, and vitamin depletion — in a single pre-sleep formula

The bottom line

A hangover isn't one problem. It's nine biological mechanisms hitting at the same time, each producing different symptoms that compound together.

This is why "drink water and hope" rarely works — it addresses one of nine causes. Effective recovery requires hitting multiple mechanisms: acetaldehyde clearance (DHM, L-cysteine), hydration with electrolytes (sodium, potassium, magnesium), B-vitamin replenishment, proper sleep, and giving inflammation time to resolve.

NUDAY Next-Day Support Gummies are built around exactly this — combining 500mg DHM, L-cysteine, B-complex, and electrolyte support into a Stage-1 (pre-sleep) formula. Taken before bed, it works alongside your body's natural overnight recovery — not after symptoms have peaked.

Try Next-Day Support Gummies →

Sources & References

1. Penning, R. et al. (2012). "The pathology of alcohol hangover." Current Drug Abuse Reviews, 5(2).
2. Verster, J.C. et al. (2010). "The Alcohol Hangover Research Group consensus statement." Current Drug Abuse Reviews, 3(2).
3. Shen, Y. et al. (2012). "Dihydromyricetin As a Novel Anti-Alcohol Intoxication Medication." Journal of Neuroscience, 32(1).
4. Eriksson, C.J.P. et al. (2020). "L-Cysteine Containing Vitamin Supplement Which Prevents or Alleviates Alcohol-related Hangover Symptoms." Alcohol and Alcoholism, 55(6).
5. Rohsenow, D.J. et al. (2010). "Intoxication with Bourbon Versus Vodka: Effects on Hangover, Sleep, and Next-Day Neurocognitive Performance." Alcoholism: Clinical and Experimental Research, 34(3).
6. Bishehsari, F. et al. (2017). "Alcohol and Gut-Derived Inflammation." Alcohol Research: Current Reviews, 38(2).
7. Hendriks, H.F.J. (2020). "Alcohol and Human Health." European Journal of Clinical Nutrition.
8. Pittler, M.H. et al. (2005). "Interventions for preventing or treating alcohol hangover: systematic review of randomised controlled trials." BMJ, 331.