You know the feeling: pounding head, queasy stomach, a mouth that tastes like the inside of a shoe. A hangover is often treated as a single, mysterious punishment for a fun night, but researchers have actually mapped out several distinct biological processes happening at once — and none of them is simply “too much alcohol” in a vague sense.

The Acetaldehyde Problem

When the liver breaks down alcohol, one of the first byproducts is acetaldehyde, a compound considerably more toxic than alcohol itself. According to the Cleveland Clinic, acetaldehyde buildup can cause a fast pulse, sweating, and nausea. The National Institute on Alcohol Abuse and Alcoholism (NIAAA) notes that acetaldehyde exposure contributes to inflammation in the liver, pancreas, brain, and gastrointestinal tract — which helps explain why a hangover feels like a body-wide event rather than something isolated to the head.

Dehydration Does Real Work Here

Alcohol suppresses vasopressin, a hormone that normally signals the kidneys to retain fluid. With that signal blunted, the body loses more water than usual through urination. NIAAA points to this fluid loss as a direct contributor to the thirst, fatigue, and headache that show up the next day. It is not the whole story, but it is a meaningful and easily addressed piece of it.

Inflammation, Stomach Irritation, and Disrupted Sleep

Alcohol increases inflammation throughout the body, which the Cleveland Clinic says contributes to the general unwell, achy feeling of a hangover. It also irritates the stomach lining and slows digestion, producing nausea and stomach upset. And while a nightcap can make it easier to fall asleep, NIAAA notes that alcohol fragments sleep and reduces time spent in restorative REM sleep — so even a full night in bed after drinking can leave someone waking up more tired than they went to sleep.

Why Some Drinks Hit Harder

Darker liquors like bourbon, whiskey, and red wine contain higher levels of congeners — byproducts of fermentation and aging that are chemically related to, but distinct from, alcohol itself. Both the Cleveland Clinic and NIAAA note that these compounds tend to make hangovers more severe than the same amount of alcohol from a clear spirit like vodka. Sulfites in wine may also trigger headaches in some sensitive individuals, according to NIAAA, adding another variable to why one drink type can feel so much worse than another.

How Long It Actually Lasts

Hangover symptoms typically begin as blood alcohol content drops significantly, which the Cleveland Clinic pegs at roughly six to eight hours after drinking — often meaning symptoms start while someone is still asleep. From there, symptoms generally ease over eight to 24 hours, with most people feeling normal again within a day. NIAAA adds that some symptoms can persist 24 hours or longer, particularly after heavier drinking, which is one reason hydration, food, and rest — rather than any quick fix — remain the most evidence-backed way to get through one.