A hangover is your body’s reaction to drinking more alcohol than it can comfortably process. It’s a combination of toxic byproducts building up in your system, dehydration, inflammation, poor sleep, and stomach irritation, all hitting at once. Symptoms peak when your blood alcohol level drops back to zero and can last 24 hours or longer.
How Your Body Breaks Down Alcohol
Understanding a hangover starts with understanding what happens to alcohol once you swallow it. Your liver does most of the heavy lifting, using an enzyme called alcohol dehydrogenase to convert ethanol into acetaldehyde. This intermediate chemical is highly toxic and a known carcinogen. Normally, a second enzyme quickly converts acetaldehyde into acetate, a harmless substance that breaks down into water and carbon dioxide. But when you drink faster than your liver can keep up, acetaldehyde lingers.
That lingering acetaldehyde causes real damage. It’s most destructive in the liver, where the bulk of alcohol processing happens, but it also affects cells in the brain, pancreas, and gastrointestinal tract. In animal studies, acetaldehyde alone produces incoordination, memory impairment, and sleepiness, effects most people blame on the alcohol itself. Some researchers believe acetaldehyde is responsible for a larger share of hangover misery than previously thought.
Why You Feel So Dehydrated
Alcohol directly suppresses the release of vasopressin, the hormone that tells your kidneys to hold onto water. Without enough vasopressin, your kidneys let far more fluid pass through than they normally would. That’s why you urinate so frequently while drinking, and why you wake up parched the next morning.
The fluid loss alone would make you feel lousy, but alcohol also changes how your body handles electrolytes. Research has shown that sodium, potassium, and chloride excretion can all shift after drinking, though the exact pattern depends on how much you consumed and your hydration level beforehand. The combined effect of lost water and disrupted electrolytes contributes to the headache, dizziness, and dry mouth that define the classic hangover experience.
Your Immune System Kicks Into Overdrive
A hangover isn’t just about toxins and dehydration. Your immune system also reacts to a night of heavy drinking. Research has found that levels of several immune-signaling molecules, specifically IL-10, IL-12, and interferon-gamma, rise significantly during a hangover compared to normal conditions. These are the same types of molecules your body releases when you’re fighting an infection, which is why a hangover can feel eerily similar to coming down with the flu.
This immune activation is closely linked to the symptoms people complain about most: nausea, headache, diarrhea, and fatigue. Your body is essentially mounting an inflammatory response to the chemical assault of the night before.
What Happens to Your Stomach
Alcohol irritates and can physically break down the protective lining of your stomach, leaving it exposed to its own digestive acids. This is a form of acute gastritis, and it’s why nausea, stomach pain, and vomiting are so common the morning after. The more you drink, the more vulnerable your stomach lining becomes. If you drank on an empty stomach, the effect is even more pronounced because there’s no food to buffer the contact between alcohol and your stomach wall.
Why You Slept Eight Hours but Feel Exhausted
Alcohol is a sedative, so it may help you fall asleep faster, but the sleep you get is poor quality. One key reason involves glutamine, a natural stimulant your brain produces. While you’re drinking, alcohol suppresses glutamine. Once you stop, your brain overcompensates by producing more glutamine than usual. This “glutamine rebound” keeps your brain in a more stimulated state throughout the night, preventing you from reaching the deeper, more restorative stages of sleep. You may technically be unconscious for hours, but your brain never fully rests.
Dark Liquors Make It Worse
Not all drinks produce equal hangovers, even at the same alcohol content. The difference comes down to congeners, chemical byproducts created during fermentation and distillation. One congener that matters most is methanol, a type of alcohol found in much higher concentrations in red wine, brandy, and whiskey than in beer or vodka.
Methanol is processed by the same liver enzyme as regular alcohol (ethanol), but your liver strongly prefers ethanol. So methanol sits waiting in line until the ethanol is cleared. Once it finally gets processed, it breaks down into formaldehyde and formic acid, both highly toxic. This delayed processing means methanol’s worst effects arrive right as your hangover is peaking. Beverages with more methanol are consistently associated with more frequent and more severe hangovers. If you’re trying to minimize the damage, clear spirits like vodka contain the fewest congeners.
The Timeline of a Hangover
Hangover symptoms don’t start while you’re still drunk. They build gradually as your blood alcohol level falls and hit their worst point right around when it reaches zero. For most people, that means symptoms peak somewhere between 12 and 16 hours after their last drink, depending on how much they consumed and how fast their body metabolizes alcohol.
From there, symptoms can persist for a full 24 hours or even longer. The headache and fatigue tend to linger longest, while nausea and stomach upset often resolve within the first half of the day. The entire experience is your body working to clear toxins, restore fluid balance, calm inflammation, and recover from disrupted sleep, all at once. There’s no shortcut through that process, which is why the only truly reliable way to avoid a hangover is to drink less in the first place.