Being hungover means your body is dealing with the toxic aftermath of processing alcohol, and it shows up as a cluster of symptoms that can disrupt your entire day. A hangover is clinically defined as the presence of at least two symptoms, including headache, fatigue, nausea, diarrhea, poor overall well-being, trembling, or loss of appetite, occurring after your body has fully metabolized the alcohol you drank. The key detail: hangovers don’t happen while you’re still drunk. They begin once the alcohol is gone and your body is left cleaning up the damage.
What’s Actually Happening in Your Body
When you drink, your liver breaks alcohol down in two stages. First, it converts alcohol into a compound called acetaldehyde, which is toxic. Then a second enzyme converts acetaldehyde into a harmless substance your body can eliminate. The problem is that acetaldehyde, the intermediate step, is the major cause of hangover symptoms. If you drink faster than your liver can complete that second conversion, acetaldehyde builds up and causes inflammation across multiple organs, including your brain, stomach, pancreas, and gastrointestinal tract.
At the same time, your immune system kicks into a low-grade inflammatory response. Levels of specific immune signaling molecules rise significantly during a hangover compared to normal conditions. This immune disruption is directly linked to the nausea, headache, diarrhea, and fatigue you feel the morning after. It’s similar to how your body responds to an infection: that general feeling of being run down and unable to function isn’t just in your head. Your immune system is genuinely activated.
Why Your Stomach Feels Wrecked
Alcohol directly irritates the lining of your stomach and triggers increased acid production. This is why nausea and stomach discomfort are among the most common hangover complaints. The irritation starts while you’re drinking but persists well into the next day. Worth noting: reaching for aspirin or ibuprofen to treat a hangover headache can actually make stomach symptoms worse, because those painkillers also increase acid release and irritate the stomach lining in the same way alcohol does.
The Mental Fog Is Real
One of the most underappreciated effects of a hangover is how much it impairs your thinking. Research consistently shows that hangovers cause measurable deficits in memory, attention, and reaction time. People in a hangover state recall fewer words on verbal fluency tests, respond more slowly to choices, and perform worse on tasks requiring sustained focus. Short-term recall for both numbers and visual patterns drops. Working memory, the mental workspace you use to hold and manipulate information, takes a hit along with your ability to inhibit impulses.
Prospective memory, your ability to remember to do things in the future (like keeping an appointment or taking medication), is also significantly impaired. In one study, hungover participants remembered substantially fewer planned tasks compared to non-hungover controls. This means a hangover doesn’t just make you feel bad. It meaningfully reduces your ability to function, make decisions, and follow through on responsibilities. If you’ve ever felt “stupid” while hungover, the cognitive data backs that up.
Why Some Drinks Hit Harder
Not all alcohol produces the same hangover, even at the same total amount consumed. The difference comes down to congeners, substances other than alcohol itself that are produced during fermentation and distilling. Congeners contribute to the flavor, color, and aroma of a drink, but they also add to the toxic load your body has to process.
Congener levels vary enormously between beverages. Red wine, brandy, and whiskey contain some of the highest concentrations, particularly of methanol, a type of alcohol your body metabolizes even more slowly than ethanol. Beer and vodka tend to have the lowest congener content. This is why a vodka hangover, drink for drink, is often milder than a bourbon hangover. You’re giving your liver fewer toxic compounds to clear.
Why Some People Get Worse Hangovers
Genetics play a significant role in how intensely you experience hangovers. The most well-studied example involves a variation in the gene that produces the enzyme responsible for breaking down acetaldehyde. About 30% of people with East Asian ancestry carry a version of this gene that produces an enzyme with dramatically reduced activity. When these individuals drink, acetaldehyde builds up much faster and lingers longer, causing facial flushing, nausea, palpitations, and general discomfort at relatively low amounts of alcohol.
This genetic variation is so reliably unpleasant that it acts as a natural deterrent. Carriers of it tend to drink less and have a lower risk of developing alcohol dependence, precisely because the immediate side effects are aversive enough to discourage heavy consumption. But genetic variation in alcohol-processing enzymes isn’t limited to one population. Differences in how quickly anyone metabolizes alcohol and its byproducts contribute to the wide range of hangover severity people experience, even when drinking comparable amounts.
How Long It Lasts
Most hangover symptoms peak when your blood alcohol level returns to zero, which for a night of heavy drinking typically means the morning after. Symptoms generally resolve within 24 hours, though severe hangovers can leave you feeling off for closer to 36 hours. Dehydration (alcohol suppresses the hormone that tells your kidneys to retain water), poor sleep quality (alcohol disrupts the later stages of sleep even if it helps you fall asleep initially), and low blood sugar all compound the inflammatory and toxic effects to create the full hangover experience.
The intensity scales with how much you drank, how quickly you drank it, whether you ate beforehand, how hydrated you were, and what you were drinking. There’s no reliable way to “cure” a hangover once it’s started. Your liver simply needs time to finish processing. Rehydrating, eating, and resting can ease symptoms, but the only guaranteed prevention is drinking less or not drinking at all.