What Causes a Hangover? Acetaldehyde, Inflammation, Sleep and Dehydration Explained

Hangovers are caused by several overlapping effects of drinking, including disrupted sleep, an inflammatory immune response, stomach irritation, fluid loss, congeners in some drinks and possibly the toxic alcohol byproduct acetaldehyde, and researchers still debate how much each contributes.^{1, 2} If you want to know what causes a hangover most reliably, it's simply how much alcohol you drank.^{8, 2} Some popular explanations, especially dehydration and low blood sugar, track hangover severity poorly when researchers actually measure them.¹

What causes a hangover? The short answer

No single thing. A hangover is the combination of negative mental and physical symptoms that can follow drinking, starting as blood alcohol approaches zero, and several processes feed into it at once.⁷ Here's how the main suspects compare:

Step one: how does your body break down alcohol?

Mostly in the liver, in two steps. The enzyme alcohol dehydrogenase (ADH) turns ethanol into acetaldehyde, and aldehyde dehydrogenase (mainly ALDH2) turns acetaldehyde into acetate. A third system, CYP2E1, does more of the work in people who drink heavily over time.⁵

About 90% of alcohol is broken down this way; less than 10% leaves the body unchanged in breath, sweat and urine.⁵ After the first few drinks, the enzymes are saturated and work at a roughly fixed rate: blood alcohol falls about 0.015% an hour in a typical moderate drinker, or about 7 grams of alcohol an hour for a 70-kg adult.^{16, 5} With 14 grams in a US standard drink, many adults need one to two hours to clear each one.⁶

That timing explains when a hangover hits. Symptoms peak as blood alcohol returns to about zero, which is why you can feel fine at 1 a.m. and awful at 10 a.m.⁸ More detail in how your body processes alcohol.

Acetaldehyde: the toxic middleman

Acetaldehyde is the toxic, short-lived compound your body makes when it first breaks down alcohol.⁸ It clearly causes symptoms when it builds up, but the evidence that it drives an ordinary hangover is weaker than most articles suggest.

The clearest picture comes from people with an inactive variant of ALDH2, the enzyme that clears acetaldehyde. About 36% of East Asians show the resulting flushing response, and the buildup causes flushing, nausea and a fast heartbeat.¹⁴ Asian Americans with the variant reported more severe hangovers.¹⁵ Flushing is also a warning sign: regular drinkers with the variant have a much higher risk of esophageal cancer, and no supplement makes drinking safer for them.¹⁴ More in Asian flush.

In everyone else, the link is murkier. A 2020 review of biomarker studies found blood ethanol and slow alcohol clearance were related to hangover severity, but the association with blood acetaldehyde was not significant.² Some individual studies do report a correlation.¹ The fair summary: acetaldehyde probably contributes, but it isn't the whole story. See what acetaldehyde is for more.

Inflammation and the immune response

Drinking sets off an immune response, and immune markers are among the most convincing biological correlates of how bad a hangover feels.¹ As NIAAA puts it, "Alcohol increases inflammation in the body."⁸

In 20 healthy men given 1.5 g of alcohol per kilogram of body weight, the immune signals IL-10, IL-12 and interferon-gamma were raised 13 hours later, during the hangover.³ In a placebo-controlled trial, the inflammation marker CRP was strongly associated with hangover severity.¹⁷ Pooled data link IL-6, TNF-α and CRP with severity as well.¹⁰

That has led some researchers to describe a hangover as inflammation-driven "sickness behavior," the tired, achy, withdrawn state that comes with an infection. It's a hypothesis, and most of the evidence is correlational.¹⁸ The gut may also play a part: long-term heavy drinking changes gut bacteria and makes the intestinal lining leakier, which promotes inflammation, though that research is mostly about chronic drinking rather than one night.¹⁹ More in alcohol, your gut and inflammation.

How does alcohol disrupt your sleep?

Alcohol is a sedative going in and a disruptor on the way out. It shortens the time it takes to fall asleep and consolidates the first half of the night, then fragments the second half, delays the first REM period and reduces total REM at moderate and high doses.⁹ NIAAA's summary: sleep is fragmented, and people tend to wake up earlier.⁸

In a lab trial of 95 heavy drinkers aged 21 to 33, alcohol reduced sleep efficiency and REM sleep and increased time awake and next-day sleepiness, and those sleep effects correlated with hangover.⁴ Fatigue is the most common hangover symptom, reported by 95.5% of students in one survey, so poor sleep is a big part of the picture.²⁰ See alcohol and sleep for the details.

Is a hangover just dehydration?

No. Dehydration is real, but it's overrated as the main cause of a hangover.

Alcohol suppresses vasopressin, a hormone that tells your kidneys to hold on to water, so you urinate more and lose extra fluid.⁸ Thirst is one of the most common hangover symptoms, reported by 89.1% of students, yet thirst and dry mouth contributed little to how bad people felt overall.²⁰

When researchers measure dehydration directly, it doesn't track hangover severity: vasopressin, electrolytes and other markers weren't significantly related to it in human studies.¹ A 2024 review called hangover and dehydration "two co-occurring but independent consequences of alcohol consumption," with dehydration effects that were usually mild and short-lived.¹¹ And in a 2026 trial, drinking water alongside sake didn't change next-day symptoms.²¹

Water still helps with thirst, and in most people, electrolyte balance recovers quickly once alcohol wears off.⁸ More in does alcohol dehydrate you?

Congeners: why bourbon may hit harder than vodka

Congeners are compounds other than ethyl alcohol that form during fermentation. Darker spirits such as bourbon tend to have more of them than clear spirits, and they can make a hangover worse for some people.⁸

In the best trial, 95 heavy drinkers drank to the same breath alcohol level (0.11%) on bourbon one night and vodka another, with placebo nights for comparison. Both drinks caused hangovers, and people felt worse after bourbon, but next-day performance didn't differ.⁴ A review by the same researchers concluded that ethanol itself has considerably stronger effects than congeners.¹²

Methanol, one congener, is cleared after ethanol, and in four volunteers its clearance coincided with the onset of hangover.²² But urine methanol didn't correlate with hangover severity in 36 drinkers.²³ Wine adds sulfites, preservatives that can cause headaches in sensitive people.⁸

Blood sugar, stomach irritation and other contributors

A few more factors are often blamed. Some hold up better than others.

• Low blood sugar. A 1998 NIAAA review proposed it as a contributor, but later human data found glucose levels weren't significantly linked to hangover severity.^{13, 1} It no longer appears in NIAAA's list of causes.⁸
• Stomach irritation. "Alcohol directly irritates the lining of the stomach and increases acid release," NIAAA says, which helps explain nausea and stomach pain.⁸
• Mini-withdrawal. As the buzz wears off, the brain rebalances, and "people can feel more restless and anxious than before they drank."⁸ A 2026 review of 22 studies found hangovers consistently linked with more anxiety, stress and low mood.²⁴ This is different from true alcohol withdrawal, which affects people who drink heavily or daily and can become life-threatening.²⁵
• Who you are. Surveys find severe hangovers are far more common in younger adults: in 51,645 Danes, men aged 18 to 29 had 6.8 times the odds of a severe hangover after binge drinking as men 60 and older.²⁶ Hangovers can also occur at blood alcohol levels below the 0.11% once assumed.²⁷

Knowing the causes also explains why remedies struggle: there isn't one mechanism to target. A 2022 review of 21 placebo-controlled trials rated all the evidence for hangover treatments as very low quality.²⁹ The surest prevention follows from the biology: less alcohol. Current US guidance says "Consume less alcohol for better overall health," and some people shouldn't drink at all, including anyone pregnant, recovering from alcohol use disorder or unable to control their drinking, or taking medicines that interact with alcohol.³⁰ A hangover can also impair attention and coordination, so don't drive until you're clear.⁸ For what does help the morning after, see how to cure a hangover.

Some symptoms aren't a hangover at all. Blood alcohol can keep rising after someone stops drinking, even while they sleep.³¹

What researchers still don't know

• How much each factor contributes, and whether inflammation is a cause of hangover symptoms or a parallel effect of drinking.¹⁸
• Why acetaldehyde, so clearly harmful when it builds up, doesn't consistently track hangover severity in blood tests.²
• Why some people seem to get few or no hangovers at the same blood alcohol levels as others.²⁷
• Whether treating any single mechanism would make a meaningful difference. No remedy has strong evidence so far.²⁹

Published by Fuller Health, which makes After Party. This article is general information, not medical advice: talk to a doctor or pharmacist about your own health, medicines and drinking. Last reviewed October 9, 2026. How we research and write.