Alcohol causes hangovers through at least six different mechanisms working simultaneously, which is why no single remedy fixes all the symptoms. When your body breaks down alcohol, it produces a toxic intermediate compound, loses excess fluid, triggers inflammation, disrupts sleep architecture, irritates your stomach lining, and throws off your blood sugar. Each of these processes contributes a different piece of the misery you feel the morning after.
Your Body Converts Alcohol Into a Toxin
The core chemistry of a hangover starts in your liver. An enzyme called alcohol dehydrogenase breaks ethanol down into acetaldehyde, a highly toxic compound and known carcinogen. Under normal circumstances, acetaldehyde is short-lived because a second enzyme quickly converts it into acetate, which your body then breaks down into water and carbon dioxide.
The problem is volume. When you drink faster than your liver can complete this two-step process, acetaldehyde accumulates. It’s reactive and damaging, particularly in the liver where most alcohol metabolism happens, but also in the brain, pancreas, and gastrointestinal tract. Animal studies have shown that acetaldehyde alone can cause incoordination, memory impairment, and sleepiness, effects we typically blame on alcohol itself. The lingering presence of this compound as your body works through a backlog of ethanol is one of the primary drivers of hangover symptoms.
Alcohol Dehydrates You From the Inside Out
Alcohol suppresses vasopressin, the hormone that tells your kidneys to retain water. Without that signal, your kidneys let far more fluid pass through than they normally would. This is why you urinate so frequently while drinking, and why total urinary output is significantly higher after alcohol compared to drinking the same volume of water. The result is a net fluid loss even if the drinks themselves contained a lot of liquid.
That fluid loss pulls electrolytes with it, contributing to the headache, dizziness, and general fatigue you feel the next morning. Your brain is especially sensitive to changes in hydration, which is why the headache tends to be the most prominent symptom.
Your Immune System Fires Up
Even a single night of heavy drinking triggers a measurable inflammatory response. Studies have found elevated levels of inflammatory markers, including interleukin-6 (IL-6), C-reactive protein, and tumor necrosis factor-alpha, in blood, saliva, and urine during the hangover state. These are the same molecules your immune system releases when you’re fighting an infection, which helps explain why a bad hangover can feel eerily similar to coming down with the flu: body aches, fatigue, brain fog, and general malaise.
Interestingly, research has not found a clean correlation between the levels of these inflammatory markers and how severe any individual person rates their hangover. Some people with high inflammation feel fine, while others with lower levels feel terrible. This suggests inflammation is one contributor among many, and individual sensitivity varies widely.
Your Sleep Is Worse Than You Think
Alcohol is sedating at first. It shortens the time it takes to fall asleep and increases deep slow-wave sleep during the first half of the night. That’s the “passing out” effect, and it can feel like solid rest in the moment.
The second half of the night tells a different story. As your blood alcohol level drops, your brain rebounds. Wakefulness increases, sleep becomes fragmented, and transitions between sleep stages become more frequent. REM sleep, the phase most important for memory consolidation and feeling mentally restored, gets suppressed in a dose-dependent way. The more you drink, the less REM sleep you get. So even if you slept for eight hours, the quality of that sleep was significantly degraded, leaving you groggy and mentally sluggish the next day.
Your Brain Overcorrects
While you’re drinking, alcohol enhances the activity of GABA, your brain’s main calming neurotransmitter, and suppresses glutamate, the main excitatory one. This is why alcohol relaxes you, slows your thinking, and lowers inhibitions.
As alcohol clears your system, the balance tips hard in the other direction. Glutamate activity surges above normal levels while GABA function drops. This rebound effect creates a state of neural hyperexcitability: your brain is essentially overcorrecting for hours of artificial calm. That’s the source of hangover anxiety (sometimes called “hangxiety”), the jittery restlessness, sensitivity to light and sound, and the racing thoughts that can accompany a morning after. It’s a mild version of the same neurochemical process that drives clinical alcohol withdrawal.
Your Stomach Takes Direct Damage
Alcohol irritates the stomach lining on contact, but the specifics depend on what you’re drinking. Beer and white wine are potent stimulants of gastric acid secretion, nearly doubling the acid response compared to a standard meal in some studies. Whisky and cognac, despite having higher alcohol concentrations, don’t stimulate acid the same way. The nonalcoholic compounds in beer and wine, not the ethanol itself, appear to be the main culprits.
Pure ethanol at low concentrations (around 1 to 4 percent) mildly stimulates acid production, while higher concentrations actually inhibit it. This is a bit counterintuitive: a strong spirit may irritate your stomach less from an acid perspective than a few beers, though the overall alcohol load still matters. The excess acid, combined with direct irritation of the stomach lining, produces the nausea, queasiness, and occasional vomiting that characterize the gastrointestinal side of a hangover.
Your Blood Sugar Drops
Your liver has two jobs competing for its attention after a night of drinking: processing alcohol and maintaining your blood sugar. Alcohol wins. The metabolic changes that occur as your liver breaks down ethanol reduce its ability to produce new glucose by roughly 45 percent in the five hours after drinking. The availability of the raw materials your liver needs to make glucose drops by about 61 percent.
Your body partially compensates by tapping into stored glycogen and adjusting how your tissues use glucose. But if your glycogen stores are already low (because you skipped dinner, for instance, or danced for hours), this compensation falls short. The result is low blood sugar, which directly affects the brain. Since glucose is the brain’s primary fuel, even a modest dip can cause shakiness, weakness, difficulty concentrating, and irritability, all hallmark hangover symptoms.
Dark Liquors Make It Worse
Not all drinks produce equal hangovers at the same alcohol dose, and the difference comes down to congeners. These are chemical byproducts of fermentation and aging: compounds like methanol, tannins, and fusel oils that give darker spirits their color and flavor. Bourbon, brandy, and red wine are high in congeners. Vodka and gin contain very little.
Research comparing bourbon and vodka at equivalent alcohol doses found that bourbon produced noticeably more severe hangovers. Beer, being darker and higher in congeners than vodka, also tends to produce worse symptoms at the same level of intoxication. Your body has to process these additional compounds through the same overloaded enzyme pathways handling ethanol, which extends and intensifies the toxic load. Methanol, for example, gets converted into formaldehyde and formic acid, both of which are harmful and slow to clear.
This doesn’t mean clear liquors are hangover-proof. Drink enough of anything and you’ll feel it. But choosing lower-congener drinks, when you’re going to drink anyway, does appear to reduce severity on the margins.