A hangover is your body’s reaction to a cascade of toxic byproducts, inflammation, and chemical disruptions triggered by heavy drinking. Symptoms typically begin as your blood alcohol level drops toward zero and peak roughly 12 to 14 hours after you started drinking. What feels like a single miserable experience is actually several biological processes hitting you at once.
How Your Body Breaks Down Alcohol
Understanding a hangover starts with understanding how your liver processes alcohol. Your liver converts ethanol into a compound called acetaldehyde, which is significantly more toxic than the alcohol itself. Normally, a second enzyme quickly converts acetaldehyde into harmless acetate, which your body can use for energy. The problem is that heavy drinking overwhelms this system, allowing acetaldehyde to build up in your bloodstream before your liver can finish the job.
Acetaldehyde buildup causes flushing, headache, nausea, vomiting, and sweating. These effects are well documented in people who genetically lack the enzyme that clears acetaldehyde. About 8% of the world’s population, predominantly people of East Asian descent, carry a mutation in the ALDH2 gene that dramatically slows the breakdown of acetaldehyde. For these individuals, even moderate drinking produces intense flushing, rapid heartbeat, nausea, and dizziness because acetaldehyde lingers in the blood far longer than usual.
Even if your enzymes work at full speed, a night of heavy drinking produces more acetaldehyde than your liver can process in real time. That temporary surplus is one of the first dominoes to fall.
The Inflammation Response
Acetaldehyde doesn’t just cause direct discomfort. As your body processes alcohol, it generates a flood of unstable molecules called free radicals. These free radicals damage cell membranes in a process called lipid peroxidation, producing byproducts that combine with acetaldehyde to form compounds your immune system treats as foreign invaders.
Your body responds the same way it would to an infection: by launching an inflammatory response. Research on healthy subjects found that during a hangover, blood levels of several immune signaling molecules, specifically IL-10, IL-12, and interferon-gamma, rise significantly compared to baseline. These are the same types of molecules responsible for the achy, fatigued, foggy feeling you get when you’re fighting off a cold or the flu. That “hit by a truck” hangover sensation isn’t just in your head. Your immune system is genuinely activated, producing many of the same symptoms as illness.
What Happens in Your Stomach
Alcohol irritates the lining of your stomach and disrupts normal digestion. Beer and wine are particularly potent stimulants of gastric acid. Research published in Gastroenterology found that adding white wine to a test meal nearly doubled the stomach’s acid output compared to the meal alone. Beer triggered even higher levels of gastrin, the hormone that signals your stomach to produce acid.
Interestingly, the culprit isn’t the alcohol itself. Pure ethanol at concentrations found in beer and wine (around 1% to 5%) is a moderate acid stimulant, but at higher concentrations like those in spirits, it actually has no stimulatory effect or even suppresses acid. The nonalcoholic compounds in beer and wine, things like fermentation byproducts and organic acids, are what drive the acid surge. This helps explain why a night of beer or wine can leave your stomach feeling more wrecked than an equivalent amount of alcohol consumed as straight spirits.
The combination of excess acid and direct irritation of the stomach lining produces the nausea, cramping, and general abdominal misery that define the gastrointestinal side of a hangover.
Blood Sugar Drops
Your liver has two main strategies for keeping your blood sugar stable: breaking down stored glucose and manufacturing new glucose from scratch. Alcohol interferes with both. While your liver is busy metabolizing ethanol, its ability to produce new glucose is suppressed, and its stored glucose gets depleted faster than normal.
The result is a dip in blood sugar that contributes to the weakness, shakiness, fatigue, and difficulty concentrating that come with a hangover. If you didn’t eat much before or during drinking, this effect is more pronounced because your liver’s glucose reserves were already low.
Why Dark Liquors Make It Worse
Not all alcoholic drinks produce equal hangovers, even at the same total alcohol content. The difference comes down to congeners: chemical byproducts created during fermentation and aging. Dark-colored spirits like bourbon and whiskey contain far higher levels of congeners than clear spirits like vodka.
A study comparing bourbon to vodka at equal alcohol doses found that bourbon produced notably more severe hangovers. One particularly problematic congener is methanol, a type of alcohol your body processes much more slowly than ethanol. Here’s the key detail: your liver prioritizes ethanol over methanol. So methanol accumulates while you’re drinking and only begins to be metabolized after the ethanol clears your system. When methanol finally gets processed, it breaks down into formaldehyde and formic acid, both of which are toxic. This delayed processing may explain why hangover symptoms often seem to worsen or linger even hours after you’ve stopped drinking. It’s also why “hair of the dog” (drinking more alcohol the next morning) can temporarily relieve symptoms: it delays methanol metabolism again, kicking the problem down the road rather than solving it.
The Hangover Timeline
Hangover symptoms don’t hit immediately. They begin as your blood alcohol concentration drops and typically start becoming noticeable around 6 to 8 hours after drinking. In controlled studies where subjects consumed a standardized dose of alcohol in the evening, hangover severity scores started climbing about 8 hours after consumption and peaked at roughly 14 hours, which for most people corresponds to the following morning.
This timing aligns with the biology. As ethanol clears, the inflammatory response is in full swing, methanol is beginning to break down into toxic byproducts, blood sugar is at its lowest, and your stomach lining has had hours of acid exposure. Everything converges at once.
The duration varies depending on how much you drank, your genetics, your body weight, and whether you ate beforehand, but most hangovers resolve within 24 hours as your body finishes clearing the backlog of toxic metabolites and the immune response subsides.
Why Some People Get Worse Hangovers
Genetics plays a larger role than most people realize. The speed at which your liver processes acetaldehyde is determined by your genes. People with the ALDH2*2 variant accumulate acetaldehyde faster and clear it slower, making even moderate drinking deeply unpleasant. But even among people with fully functional enzymes, there’s natural variation in how efficiently the entire detoxification chain operates.
Beyond genetics, several practical factors influence severity. Drinking on an empty stomach accelerates alcohol absorption, giving your liver less time to keep up. Dehydration compounds the headache and fatigue, since alcohol suppresses the hormone that tells your kidneys to retain water. Choosing drinks high in congeners adds an extra layer of toxic byproducts to process. And sleep disruption, which alcohol reliably causes even if you feel like you passed out easily, makes every other symptom feel worse by depriving your brain of the restorative sleep stages it needs to recover.
A hangover, in short, isn’t one thing. It’s a pileup of at least half a dozen overlapping biological insults, each with its own mechanism, all converging in the hours after your blood alcohol hits zero.