What Actually Causes a Hangover?

A hangover is not just dehydration. It is a pile-up: fluid loss, broken sleep, stomach irritation, inflammation, alcohol byproducts, and a brain rebound after the buzz wears off.

That is why one fix rarely changes the whole morning. A hangover feels broad because the causes are broad.

What is a hangover, exactly?

NIAAA describes a hangover as a set of symptoms after drinking too much. The list can include fatigue, weakness, thirst, headache, muscle aches, nausea, stomach pain, vertigo, sensitivity to light and sound, anxiety, irritability, sweating, and increased blood pressure.

That long list is the clue: several systems are recovering at once, not one organ complaining.

If you are looking for whether anything cures it, that is the companion question: how to cure a hangover. This page is about why the hangover happens in the first place.

Is dehydration the main cause?

Dehydration is real, but it is one cause among several.

NIAAA says alcohol suppresses vasopressin, a hormone that helps the kidneys hold on to fluid, so drinking can make you urinate more and lose fluid. A controlled crossover trial also found that alcoholic beverages can have a diuretic, or fluid-losing, effect.

That explains the thirst, dry mouth, and some of the headache. It does not explain everything else. You can rehydrate and still feel foggy, nauseated, anxious, raw, or unable to focus. That is because the hangover is still happening through other pathways.

Why does sleep feel useless after drinking?

Alcohol can make sleep look easier at first and worse later.

NIAAA includes disrupted and fragmented sleep among the factors behind hangover symptoms. Peer-reviewed sleep research describes the same pattern: alcohol may help people fall asleep faster, but it disrupts and fragments sleep later in the night, reducing sleep quality.

That is why a "full night" after drinking can still feel thin. The clock may say you slept. Your brain may have spent the second half of the night waking, shifting, sweating, or missing the steadier sleep that makes morning feel like morning.

This is also why the hangover can feel emotional. Poor sleep makes the next day louder: light is harsher, irritation is closer, and anxiety can feel out of proportion to the facts.

What is happening in the stomach and immune system?

Alcohol can irritate the stomach lining and increase stomach acid, which helps explain nausea, stomach pain, and that sour, unsettled feeling. NIAAA also names increased inflammation as one part of the hangover picture.

Inflammation is a broad word, but the plain version is this: your body treats alcohol exposure and its aftermath as something to respond to. That response can show up as aches, malaise, headache, and the heavy-body feeling that makes a hangover feel like more than being tired.

The body is doing cleanup. Cleanup has a cost.

What is acetaldehyde?

Acetaldehyde is a toxic byproduct your body makes as it breaks down alcohol. NIAAA includes exposure to acetaldehyde among the overlapping causes of hangover symptoms.

You do not need the chemistry to understand the point. Alcohol does not simply pass through as a mood change and leave. The body has to process it, and some of what gets produced along the way is unpleasant and stressful.

That processing time is one reason a hangover cannot be forced to end on demand.

Why does it peak when the alcohol is gone?

This is the part that surprises people. A hangover often peaks when blood alcohol returns to about zero.

NIAAA says hangover symptoms peak when blood alcohol concentration returns to about zero and can last 24 hours or longer. That timing is why you may feel worse after the "drunk" part is over. The body is no longer under the same alcohol effect, and the brain is readjusting.

NIAAA calls part of this a "mini-withdrawal" rebound as the brain shifts back after the buzz wears off. That does not mean every hangover is alcohol withdrawal in the medical sense. It means the brain can feel jangly, anxious, irritable, and off as alcohol leaves.

If that rebound includes shaking, sweating, panic, nausea, or feeling unwell whenever alcohol wears off after heavy daily drinking, do not treat it as a normal hangover. That is a reason to talk with a clinician about withdrawal safety before cutting back hard.

So how do you tell which one it is? You may not be able to know from symptoms alone. A hangover follows drinking too much and gradually passes as the body recovers. Withdrawal is more concerning when symptoms appear as alcohol wears off after regular heavy use, or when another drink seems to settle the symptoms. If that pattern sounds familiar, the safer label is "medical question," not "bad hangover."

Do congeners or sulfites matter?

They can for some people.

NIAAA notes that congeners, which are compounds other than ethyl alcohol produced during fermentation and found in higher amounts in some darker spirits, could worsen hangover symptoms for some people. It also says sulfites added to wine may trigger a headache in people who are sensitive to them.

This is not a promise that switching drinks solves the problem. It is a reason some people notice that bourbon, red wine, or certain wines feel worse than the same number of lighter drinks.

If you compare drinks, compare standard drinks, not glasses. NIAAA describes a U.S. standard drink as 0.6 fluid ounces, or 14 grams, of pure alcohol. A larger pour can make a "same amount" comparison less true than it feels.

The answer next time

NIAAA is plain that there is no cure for a hangover other than time. That matters here because knowing the causes can prevent the wrong hunt. The fix is not finding the perfect morning trick for dehydration, sleep, stomach acid, inflammation, acetaldehyde, and rebound all at once.

The reliable lever is upstream: less alcohol, slower drinking, stopping earlier, or not drinking. That is just where the mechanism points, not a moral lesson.

The other useful lever is pattern memory. If red wine reliably means headache, late drinks mean 3 a.m. anxiety, or fast drinking means nausea, write that down before the details soften. A hangover can be data without becoming self-punishment.

For related reading, see how long hangovers last, alcohol and headaches the day after, does alcohol dehydrate you, and why am I so tired after drinking.

FAQ

Is a hangover just dehydration?

No. Dehydration is one part, but NIAAA also points to fragmented sleep, stomach irritation, inflammation, acetaldehyde, and a mini-withdrawal rebound.

Why do hangovers feel worse when I sober up?

NIAAA says symptoms peak when blood alcohol returns to about zero. The body is processing alcohol byproducts and the brain is readjusting after alcohol leaves.

Do darker drinks cause worse hangovers?

They can for some people. NIAAA notes that congeners, found in higher amounts in some darker spirits, may worsen symptoms for some people.

This article is general education, not a diagnosis or cure protocol; severe, unusual, prolonged, or withdrawal-like symptoms after drinking should be discussed with a licensed clinician or urgent care.