A hangover is the result of several overlapping biological disruptions, not a single cause. When you drink alcohol, your body treats it as a mild poison, and the process of breaking it down triggers dehydration, immune activation, blood sugar drops, poor sleep, and stomach irritation all at once. Symptoms typically peak right around the time your blood alcohol level falls back to zero, which is why you feel worst the morning after rather than while you’re still drinking.
Dehydration and Fluid Loss
Alcohol suppresses the hormone that tells your kidneys to hold onto water. Normally, this hormone (called vasopressin) keeps your body from losing too much fluid. When alcohol pushes it down, your kidneys release more dilute urine than they should, and you lose more liquid than you’re taking in. Studies comparing alcohol to water intake found significantly higher total urine volume in every alcohol group, even at moderate doses.
What makes this worse is the timing. The initial suppression of vasopressin lasts longer with alcohol than it does after drinking water. Hours later, your body overcorrects by ramping the hormone back up and retaining water, which can contribute to the bloated, puffy feeling the next day. The net fluid loss in between explains the thirst, dry mouth, dizziness, and headache that define the classic hangover.
Your Body’s Toxic Middleman: Acetaldehyde
Your liver breaks alcohol down in two steps. First, it converts ethanol into acetaldehyde, a compound that is significantly more toxic than alcohol itself. Then a second enzyme converts acetaldehyde into harmless acetate. If that second step works efficiently, acetaldehyde clears your system quickly and you feel relatively little damage. If it doesn’t, acetaldehyde accumulates and directly damages living cells.
About 36% of East Asian populations carry a genetic variant that makes that second enzyme sluggish or nonfunctional. For these individuals, even small amounts of alcohol cause facial flushing, nausea, rapid heartbeat, and drowsiness, all driven by acetaldehyde building up faster than the body can clear it. But even people with fully functional enzymes experience some degree of acetaldehyde exposure during heavy drinking, because the sheer volume of alcohol overwhelms the liver’s processing speed. That temporary backup is one reason hangover severity scales with how much you drink.
An Immune System on Alert
Hangovers involve a measurable immune response. Research comparing blood samples during a hangover to baseline found significant increases in three immune signaling molecules: IL-10, IL-12, and interferon-gamma. These are the same types of compounds your body releases when fighting an infection, and they produce familiar symptoms: headache, nausea, fatigue, and diarrhea.
This helps explain why a hangover can feel so much like being sick. Your body is mounting a low-grade inflammatory response to the metabolic damage caused by alcohol, using the same pathways it would activate against a virus. The worse the drinking episode, the stronger this immune reaction tends to be.
Blood Sugar Drops
Your liver has two jobs competing for its attention after a night of drinking: maintaining your blood sugar and detoxifying alcohol. It prioritizes the alcohol. Normally, your liver steadily releases stored glucose between meals and overnight to keep your energy stable. When it’s busy processing ethanol instead, that glucose supply slows or stops, and your blood sugar can fall to abnormally low levels.
The symptoms of low blood sugar overlap heavily with the symptoms of being drunk: shakiness, weakness, confusion, drowsiness, and difficulty concentrating. This is part of why a hangover can feel like a fog you can’t think through. It’s also why eating before and during drinking tends to reduce hangover severity. Food gives your body an alternative glucose source while your liver is occupied.
Stomach Irritation and Nausea
Alcohol directly stimulates acid production in your stomach. Even relatively low concentrations (around 4 to 5%) significantly increase gastric acid secretion, pushing output to more than half of what your stomach produces at maximum capacity. The mechanism doesn’t rely solely on the hormone gastrin. Alcohol appears to stimulate acid-producing cells through multiple pathways, including direct contact with the stomach lining and histamine release.
Wine is a particularly strong trigger because it contains organic acids (like succinic and maleic acid) that independently boost acid production on top of the alcohol itself. This excess acid irritates and inflames the stomach lining, which is why nausea, cramping, and sometimes vomiting are such consistent hangover symptoms. If you’ve ever noticed that wine or cocktails made with whiskey hit your stomach harder than beer or vodka, the acid content of the drink itself is part of the reason.
Why Dark Liquors Make It Worse
Not all alcoholic drinks produce equal hangovers. Dark spirits like bourbon, brandy, and whiskey contain higher levels of compounds called congeners, which are chemical byproducts of fermentation and aging. Methanol is one of the most significant. Your body processes methanol using the same enzymes it uses for ethanol, but methanol breaks down into formaldehyde and formic acid, both of which are toxic.
Red wine and aged spirits consistently show the highest methanol concentrations, while beer and vodka contain the least. Studies have confirmed that beverages with more methanol are associated with more frequent and more severe hangovers. This doesn’t mean clear spirits won’t give you a hangover, but the additional toxic load from congeners in darker drinks adds a measurable layer of misery.
Wrecked Sleep
Alcohol might help you fall asleep faster, but it ruins the quality of the sleep you get. Once alcohol is in your system, your brain cycles through brief awakenings throughout the night, fragmenting your sleep architecture. Each of these micro-awakenings resets you to a lighter sleep stage, cutting into the deep, restorative REM sleep your brain needs to recover.
REM sleep typically pays the heaviest price. Since most REM sleep occurs in the second half of the night, right when your body is actively metabolizing the last of the alcohol, the disruption hits during the sleep stages that matter most for feeling rested. This is why you can sleep eight or nine hours after drinking and still wake up exhausted. The hours were there, but the actual restorative sleep was not.
Oxidative Stress and Cellular Damage
The process of metabolizing alcohol generates free radicals, unstable molecules that damage cells. Animal research has shown that a single dose of alcohol can deplete the DNA inside mitochondria (the energy-producing structures in your cells) by roughly 50% across the liver, heart, skeletal muscles, and brain. In most organs, this damage peaked within two hours. In the brain, it took closer to ten hours, which lines up with the timeline of hangover symptoms.
When researchers blocked alcohol metabolism with an inhibitor, the DNA damage didn’t occur, confirming that it’s the breakdown process, not alcohol itself, that causes the oxidative harm. Antioxidants like vitamin E and coenzyme Q partially reduced the damage in these studies. This oxidative stress contributes to the overall feeling of fatigue, weakness, and malaise that makes a hangover feel like your entire body is running at half capacity, because at a cellular level, it is.
Why Symptoms Peak When You’re “Sober”
One of the counterintuitive things about hangovers is that they feel worst after the alcohol has already left your system. Symptoms typically begin as blood alcohol concentration drops toward zero and intensify from there. This makes sense once you understand that hangover symptoms aren’t caused by alcohol in your blood. They’re caused by the aftermath: the dehydration that’s already happened, the acetaldehyde your liver produced, the immune response already underway, the depleted blood sugar, and the disrupted sleep you just woke up from.
By the time you feel the hangover, the damage is already done. Your body is dealing with the cumulative consequences of everything that happened while you were drinking, not the alcohol itself. This is also why there’s no reliable shortcut to cure a hangover. The multiple overlapping mechanisms mean no single remedy addresses all of them at once. Rehydrating helps with fluid loss, eating helps with blood sugar, and time helps with the rest.