Being hungover is the collection of physical and mental symptoms you experience the day after drinking alcohol, once your body has fully processed the alcohol out of your system. The medical term is veisalgia, and it kicks in as your blood alcohol concentration drops toward zero. Despite how common hangovers are, no single mechanism explains them. Instead, several overlapping processes in your body create that familiar combination of headache, nausea, fatigue, and brain fog.
What Happens Inside Your Body
Your liver breaks down alcohol in two steps. First, it converts alcohol into an intermediate substance called acetaldehyde. Then a second enzyme converts acetaldehyde into acetate, which is harmless. The problem is that acetaldehyde is toxic. Even at low concentrations it binds to proteins and other important molecules in your body. At higher concentrations, it triggers a rapid pulse, sweating, skin flushing, nausea, and vomiting.
In most people, the liver clears acetaldehyde quickly enough that it never builds up to dangerous levels. But its toxic effects can linger into the next day, even after it’s no longer detectable in the blood. Some people carry a genetic variant that makes them much slower at clearing acetaldehyde. These individuals flush, sweat, and feel sick after even small amounts of alcohol, essentially experiencing an amplified version of hangover symptoms while they’re still drinking.
Your Immune System Fires Up
A hangover isn’t just about alcohol metabolism. Your immune system reacts to a night of heavy drinking much the way it responds to an infection: by releasing signaling molecules that promote inflammation. Research measuring these molecules in saliva found that levels of two key inflammatory markers roughly doubled the morning after drinking compared to a control day without alcohol. A third marker, associated with broader inflammation throughout the body, also rose significantly.
This immune response helps explain why a hangover can feel eerily similar to coming down with something. The headache, body aches, fatigue, and difficulty concentrating overlap heavily with what you’d feel during a mild flu. Your body is genuinely inflamed, not just tired.
Dehydration Is Part of It, Not All of It
You’ve probably heard that hangovers are “just dehydration.” Alcohol does cause you to lose more water than you take in. It suppresses vasopressin, the hormone that tells your kidneys to hold onto water. When vasopressin drops during drinking, your kidneys let more fluid pass through, which is why you urinate so frequently after a few drinks. That fluid loss contributes to thirst, dry mouth, dizziness, and lightheadedness the next morning.
But dehydration alone doesn’t account for the full picture. Studies have found that rehydrating doesn’t reliably eliminate hangover symptoms. The nausea, cognitive fog, and mood changes involve inflammation, disrupted sleep, and metabolic shifts that water alone can’t fix.
Blood Sugar Drops and Sleep Falls Apart
Alcohol interferes with your liver’s ability to produce glucose, the sugar your brain depends on for energy. It suppresses a process called gluconeogenesis, where the liver manufactures new glucose, and may also increase insulin secretion. The result is a dip in blood sugar that can leave you feeling shaky, weak, irritable, and mentally foggy the next morning. It also explains the intense hunger many people feel during and after drinking.
Sleep takes a hit too, even if you feel like you passed out easily. Alcohol shortens the time it takes to fall asleep, but it wrecks the second half of the night. Once your body starts processing the alcohol, sleep becomes fragmented. You wake up more often, spend more time in the lightest stage of sleep, and lose the deeper, restorative phases your brain needs. This is why you can sleep for eight or nine hours after drinking and still wake up exhausted. That poor sleep quality feeds directly into the concentration problems, irritability, and sluggishness that define a hangover.
Why Some Drinks Feel Worse
Not all alcoholic drinks produce the same hangover. Darker spirits like bourbon, brandy, and red wine contain higher levels of congeners, which are chemical byproducts created during fermentation and aging. One congener in particular, methanol, is broken down by the same enzymes that process regular alcohol, but it produces formaldehyde and formic acid as byproducts. Because your body prioritizes clearing ethanol first, methanol lingers in your system longer, extending the window of toxic byproduct exposure. Lighter drinks like vodka, gin, and white wine contain fewer congeners and tend to produce milder hangovers at equivalent alcohol doses.
The Timeline of a Hangover
Hangover symptoms begin as your blood alcohol concentration approaches zero, not while you’re still intoxicated. For most people, that means symptoms start in the early morning hours and peak sometime before midday. The severity and duration depend on how much you drank, how quickly, whether you ate beforehand, your body weight, your genetics, and your overall health. A typical hangover lasts anywhere from 12 to 24 hours, though heavy drinking episodes can leave you feeling off for longer.
As you age, hangovers tend to feel worse. This is partly because your body’s enzyme activity and recovery capacity change over time, and partly because sleep quality generally declines with age, making the alcohol-related sleep disruption hit harder.
No Proven Cure Exists
Despite an enormous market for hangover remedies, no treatment has been shown to work in independent, double-blind, placebo-controlled trials. That includes electrolyte drinks, B vitamins, herbal supplements, and “detox” products. Some of these may address individual symptoms. Rehydrating helps with thirst and dizziness. Anti-inflammatory pain relievers can ease a headache. Eating can stabilize blood sugar. But nothing reliably shortens or eliminates the full syndrome.
The only factor consistently linked to hangover severity is the amount of alcohol consumed. Drinking less, drinking slowly, eating before and during, and alternating alcoholic drinks with water all reduce the load your body has to process. That’s not a cure, but it’s the closest thing to one that the evidence supports.