What Causes a Hangover? Sleep, Inflammation and Dehydration

A hangover is "the combination of negative mental and physical symptoms which can be experienced after a single episode of alcohol consumption, starting when blood alcohol concentration (BAC) approaches zero", and it is not one thing.¹ The best-measured causes are broken sleep, inflammation and the alcohol itself; acetaldehyde, blood sugar and oxidative stress rest on weaker or null findings; dehydration runs alongside the hangover, not underneath it. Part of how to not be hungover, the Wellgard guide.

What is a hangover, and when does it start?

That is the Alcohol Hangover Research Group's 2020 definition. It dropped "heavy", because hangovers are "more likely when subjects consume more alcohol than they usually do", and "the day after", because one "can start at any time of day or night, whenever BAC approaches zero".¹

Which symptoms matter most?

Among 1,410 students, the commonest symptoms were fatigue (95.5%) and thirst (89.1%), yet when 47 symptoms were factor-analysed, "drowsiness" explained 28.8% of the variance, "cognitive problems" 5.9% and "disturbed water balance" under 5%.² In a second survey of 1,837, the four symptoms with the biggest effect on mood, thinking and function were being tired, sleepiness, headache and concentration problems; thirst had little impact.³ The anxious, low feeling has its own page: hangover anxiety.

What is happening in the body?

Nine things have been measured against how bad people felt; the grade (A strong, B moderate, C weak) rates the literature, never an ingredient.

Inflammation, briefly: in a re-analysis of two controlled studies, severity correlated with blood interleukin-6, tumour necrosis factor-alpha and C-reactive protein, and at four hours ethanol, not acetaldehyde, went with the IL-6 rise.⁴ Oxidative-stress markers rose too, and in a 214-person trial a vitamin, mineral and antioxidant supplement "did not lead to a statistically significant improvement in hangover symptoms".^{4, 5}

Is a hangover just dehydration?

Not in any study that measured both. The 2010 review of blood and urine studies: hormones, electrolytes, glucose and cortisol did not correlate with severity, and "markers of dehydration (e.g., vasopressin) were not significantly related to hangover severity".⁶ The 2024 review: "alcohol hangover and dehydration are two co-occurring but independent consequences of alcohol consumption"; hangovers "were typically relatively enduring", dehydration "usually mild and short-lasting", and water drunk once hungover "was not related to changes of hangover severity and thirst".⁷ Six men who drank six pints in three hours had normal water balance again by nine hours: "All subjects had a slight hangover, but none was fluid depleted."⁸ Thirst is real and short-lived; the hangover outlasts it. NHS fluid wording: what to eat and drink with a hangover.

What does alcohol do to your sleep?

Sleep recordings are the strongest evidence in the field. The review of every laboratory study in healthy volunteers: "At all dosages, alcohol causes a reduction in sleep onset latency, a more consolidated first half sleep and an increase in sleep disruption in the second half of sleep".⁹ Among 578 students, those who slept under 7 hours reported more severe hangovers; longer sleepers had drunk more and reported longer but milder ones.¹⁰ The second-half waking: why alcohol wakes you up at 3am.

Does acetaldehyde cause the hangover?

Less than its reputation suggests. The 2020 review of alcohol-metabolism studies found "a direct relationship between blood ethanol concentration and hangover severity, whereas this association is not significant for acetaldehyde", and that faster ethanol elimination went with milder hangovers.¹¹ Clearance times: how long alcohol stays in your system.

It matters for one group: carriers of an inactive ALDH2 gene, common in East Asia and rare in the UK, flush when they drink, and among 251 Japanese adults "the amount of drinking reported to lead to hangover was significantly less".¹² For everyone else the blinded data point the other way: in a 40-person double-blind placebo-controlled crossover of a glutathione yeast extract, serum acetaldehyde was significantly lower at every time point, yet "there was no significant difference between the two groups" on the two hangover scales; three authors were employed by the maker.¹³ The other ingredient trials are on what the trials of hangover pills and supplements found.

Do darker drinks give worse hangovers, and does drink order matter?

One controlled trial exists: 95 heavy drinkers drank bourbon one night and vodka another, to 0.11 g% breath alcohol, against placebo. "No effect of beverage congeners was found except on hangover severity, with people feeling worse after bourbon"; next-day performance and sleep did not differ, and "Congener content affects only how people feel the next day".¹⁴ No trial of drink order or of soju was found. Vodka against wine, soju, whichever drink is called the worst: amount first, type a distant second, and the NHS is plain that "there's no safe drinking level".¹⁵

Why do you feel sick, cold or hungry the day after?

The records describe these better than they explain them: hunger, chills and changes in the stools were not measured in any study found, and the surveys record reduced appetite rather than hunger.³ Blood sugar has been tested: in 109 fasting men given 1.75 g of ethanol per kilogram, the ethanol-only group had low glucose and "a marked metabolic acidosis during hangover", yet fructose or glucose corrected the blood changes and "do not affect the symptoms or signs".¹⁶

Why do some people never get one?

A 2008 review concluded that "around 23% of the population may be resistant to hangover"; why is unknown.¹⁷ Part is inherited: in 4,496 Australian twins, genes accounted for 45% of the variation in hangover frequency in men, 40% in women, and 43% of the variation in resistance.¹⁸ Age and the other differences: why your hangovers feel so bad.

Frequently asked questions

Can you feel hungover without drinking?

Not in the research sense: the definition requires "a single episode of alcohol consumption".¹ Persistent tiredness without drinking is a GP question.

Why does a hangover headache feel like that?

In 32 volunteers across 96 episodes, a headache came with 57.3% of hangovers: frontal, pressing, worse with activity and on standing (89.1%); a history of headaches raised the odds (odds ratio 3.48).¹⁹

Does the inflammation finding mean an anti-inflammatory is the answer?

This page does not advise on medicines. The NHS sentences on paracetamol and ibuprofen with alcohol are on the hangover pills page; ask a pharmacist.

If you are worried about your drinking

"Drinkline is the national alcohol helpline. If you're worried about your own or someone else's drinking, you can call this free helpline in complete confidence. Call 0300 123 1110 (weekdays 9am to 8pm, weekends 11am to 4pm)." "A good place to start is with a GP." (NHS, Alcohol support, reviewed 17 August 2026.)²⁰ In Scotland, NHS inform lists Drinkline Scotland on 0800 7314 314.²¹ "Call 999 if: you think someone has alcohol poisoning; you or someone else has had a seizure or fit; someone has lost consciousness; someone has stopped breathing." (NHS, Alcohol poisoning, reviewed 4 June 2026.)²²

The NHS guidance: "men and women are advised not to drink more than 14 units a week on a regular basis", "spread your drinking over 3 or more days if you regularly drink as much as 14 units a week" and "try to have several drink-free days each week". "If you drink less than 14 units a week, this is considered low-risk drinking. It's called 'low risk' rather than 'safe' because there's no safe drinking level." (NHS, The risks of drinking too much, reviewed 10 September 2026.)¹⁵

Sources

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5. Lieb B, Schmitt P. Randomised double-blind placebo-controlled intervention study on the nutritional efficacy of a food for special medical purposes and a dietary supplement in reducing the symptoms of veisalgia. BMJ Nutr Prev Health 2020. PubMed 33235969
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12. Yokoyama M et al. Hangover susceptibility in relation to aldehyde dehydrogenase-2 genotype, alcohol flushing, and mean corpuscular volume in Japanese workers. Alcohol Clin Exp Res 2005. PubMed 16046871
13. Song G et al. Effects of GSH on alcohol metabolism and hangover improvement in humans: a randomized double-blind placebo-controlled crossover clinical trial. Nutrients 2024. PubMed 39408229
14. Rohsenow DJ et al. Intoxication with bourbon versus vodka: effects on hangover, sleep, and next-day neurocognitive performance in young adults. Alcohol Clin Exp Res 2010. PubMed 20028364
15. NHS. The risks of drinking too much. Page last reviewed 10 September 2026. nhs.uk/live-well/alcohol-advice/the-risks-of-drinking-too-much
16. Ylikahri RH et al. Effects of fructose and glucose on ethanol-induced metabolic changes and on the intensity of alcohol intoxication and hangover. Eur J Clin Invest 1976. PubMed 1253812
17. Howland J, Rohsenow DJ, Edwards EM. Are some drinkers resistant to hangover? A literature review. Curr Drug Abuse Rev 2008. PubMed 19630704
18. Slutske WS et al. Genetic influences on alcohol-related hangover. Addiction 2014. PubMed 25098862
19. García-Azorín D et al. Will I have a hangover headache tomorrow? A prospective cohort study of the predictors of delayed alcohol-induced headache. Life 2025. PubMed 41302147
20. NHS. Alcohol support. Page last reviewed 17 August 2026. nhs.uk/live-well/alcohol-advice/alcohol-support
21. NHS inform. Alcohol support. Page last updated 7 October 2026.. nhsinform.scot/healthy-living/alcohol/alcohol-support
22. NHS. Alcohol poisoning. Page last reviewed 4 June 2026. nhs.uk/conditions/alcohol-poisoning