A hangover is your body’s reaction to processing alcohol and dealing with the damage it leaves behind. There’s no single cause. Instead, several overlapping biological mechanisms hit you at once: toxic byproducts of alcohol metabolism, dehydration, immune system activation, disrupted sleep, and irritation of your stomach lining. Hangover symptoms peak when your blood alcohol concentration drops back to about zero, which is why you feel worst the morning after rather than while you’re still drinking.
How Your Body Breaks Down Alcohol
The core chemistry behind a hangover starts in your liver. Your body treats alcohol as a poison and works to eliminate it in two steps. First, an enzyme converts alcohol into acetaldehyde, a highly toxic substance and known carcinogen. Then a second enzyme breaks acetaldehyde down into acetate, which is harmless and eventually leaves the body as water and carbon dioxide.
The problem is that first step happens faster than the second. While acetaldehyde is usually short-lived, it’s potent enough to cause real damage during its brief existence, particularly in the liver where most alcohol processing happens. Smaller amounts of alcohol are also broken down in the gastrointestinal tract and even the brain, exposing those tissues to acetaldehyde directly. This toxic intermediate is responsible for several classic hangover symptoms: rapid pulse, sweating, and nausea.
Some people process acetaldehyde much more slowly due to genetic differences in their enzymes. People who carry a low-activity variant of the enzyme that clears acetaldehyde experience facial flushing and discomfort after even small amounts of alcohol, because the toxin builds up faster than their body can remove it. This is most common in East Asian populations and effectively makes hangovers more intense and immediate.
Dehydration and Fluid Loss
Alcohol suppresses a hormone called vasopressin (also known as antidiuretic hormone), which normally tells your kidneys to hold on to water. When vasopressin drops, your kidneys release far more water than usual. That’s why you urinate so frequently while drinking. The result is a net loss of fluids that your body doesn’t automatically replace, even if you’re drinking liquid all night, because most of what you’re consuming is triggering further fluid loss.
This dehydration contributes directly to the headache, dry mouth, dizziness, and fatigue you feel the next day. Your blood volume decreases, blood flow to the brain is affected, and your body is working to restore electrolyte balance with fewer resources than it needs.
Your Immune System Fires Up
One of the less obvious causes of a hangover is inflammation. Research on healthy subjects found that levels of several immune signaling molecules, specifically IL-10, IL-12, and IFN-gamma, were significantly elevated during the hangover state compared to normal conditions. These are the same types of molecules your body produces when you’re fighting an infection, which is why a bad hangover can feel remarkably similar to the flu: body aches, fatigue, brain fog, and general malaise.
This inflammatory response isn’t just a side effect. Researchers believe the disrupted immune signaling pathway is directly associated with hangover symptoms, not merely correlated with them. Your body is essentially mounting a low-grade immune response to the damage alcohol caused.
Why Darker Drinks Make It Worse
Not all alcoholic drinks produce equally bad hangovers, and the reason comes down to compounds called congeners. These are chemical byproducts created naturally during fermentation and distilling. The congener that matters most is methanol, a type of alcohol your body processes even more slowly than ethanol.
Methanol concentrations are highest in red wine and dark spirits like brandy and whiskey, and lowest in beer and vodka. When your body finally gets around to breaking down methanol, it produces formaldehyde and formic acid, both highly toxic. The timing is telling: methanol elimination from the body coincides with hangover onset, suggesting these toxic byproducts are piling onto the misery right as your other hangover mechanisms are kicking in.
This is why the old advice to stick with clear liquors has some basis in biology. Vodka and gin contain fewer congeners than bourbon or scotch, and studies have consistently found that higher-congener drinks are associated with more frequent and more severe hangovers.
Blood Sugar Drops
Alcohol interferes with your liver’s ability to produce and release glucose, the sugar your brain and muscles depend on for energy. Normally, your liver can manufacture new glucose or release it from stored reserves. Alcohol disrupts both of these processes. The result can be a drop in blood sugar, especially if you were drinking on an empty stomach or if your glycogen stores were already low.
This low blood sugar contributes to the shakiness, weakness, fatigue, and difficulty concentrating that characterize a hangover morning. Your brain is particularly sensitive to glucose levels, which is why cognitive function takes such a noticeable hit.
Stomach and Gut Irritation
Alcohol is a direct irritant to the lining of your stomach and intestines. It slows digestion and increases acid production, which is why nausea, stomach pain, and sometimes vomiting are among the earliest and most persistent hangover symptoms. This isn’t just about the volume of liquid you consumed. Alcohol chemically disrupts the protective mucous layer of your digestive tract, leaving the tissue underneath exposed to acid.
Disrupted Sleep
Even if you passed out for eight hours, alcohol-fueled sleep is not restorative sleep. Alcohol specifically suppresses REM sleep, the deep, dream-heavy stage that your brain needs for memory consolidation and mental recovery. Your sleep becomes fragmented, meaning your brain briefly wakes up and interrupts your sleep cycle repeatedly throughout the night. Each of these micro-awakenings can send you back to the lightest stage of sleep, cutting into the deeper stages your body needs.
Alcohol also worsens sleep apnea, a condition where breathing temporarily stops during sleep. Even people who don’t normally have sleep apnea can experience episodes after heavy drinking, adding further fragmentation. The end result is that you wake up exhausted despite technically being unconscious for a full night. This poor sleep quality amplifies every other hangover symptom, from impaired concentration to irritability to physical fatigue.
Why Some People Get Worse Hangovers
Individual variation in hangover severity is real and rooted in biology. The biggest factor is how efficiently your body clears acetaldehyde. People with genetic variants that produce a less active version of the acetaldehyde-clearing enzyme accumulate more of this toxin for longer, leading to worse symptoms. These individuals often experience flushing, nausea, and rapid heartbeat even from moderate drinking, which typically leads them to drink less overall.
Beyond genetics, several other factors influence severity. Drinking on an empty stomach accelerates alcohol absorption and worsens the blood sugar crash. Dehydration before you start drinking means you have less buffer against fluid loss. Body weight, biological sex, and how frequently you drink all affect how quickly you metabolize alcohol. And mixing different types of drinks can increase your total congener exposure, compounding the toxic load your body has to process.
A standard drink in the United States contains about 14 grams of pure alcohol, whether that’s a 12-ounce beer, a 5-ounce glass of wine, or a 1.5-ounce shot of spirits. Hangover risk increases with every standard drink beyond what your liver can process in a given timeframe, and that processing rate is relatively fixed regardless of tolerance or experience.