A hangover is not one thing going wrong in your body. It’s several overlapping problems hitting at once: inflammation, nervous system rebound, dehydration, disrupted sleep, and irritation of your gut lining. Symptoms typically peak the morning after heavy drinking, right around the time your blood alcohol level drops to zero. Understanding what’s actually happening inside you explains why hangovers feel so uniquely miserable and why no single “cure” can fix all of it.
Your Immune System Treats Alcohol Like a Threat
One of the biggest drivers of hangover misery is inflammation. Drinking triggers your immune system to release signaling molecules called cytokines, the same ones your body produces when you’re fighting an infection. Research published in the Journal of Clinical Medicine found that hangover severity correlated directly with blood levels of specific inflammatory markers, particularly one called IL-6 and another called TNF-alpha. The stronger the inflammatory response, the worse people felt.
Interestingly, it’s the alcohol itself (ethanol) that appears to drive this inflammation, not its toxic byproduct acetaldehyde. At four hours after drinking, blood ethanol concentration was significantly associated with elevated IL-6 levels, and those levels remained elevated at the 12-hour mark, well into hangover territory. This inflammatory cascade is why a hangover can feel so much like being sick: the fatigue, body aches, headache, and brain fog share the same underlying biology as a mild flu.
Your Nervous System Overcorrects
Alcohol is a depressant. It works by enhancing the activity of your brain’s main “calm down” chemical (GABA) while suppressing the main “speed up” chemical (glutamate). The result is that relaxed, loosened-up feeling. But your brain doesn’t just sit there passively. While you’re drinking, it’s actively pushing back, dialing down its sensitivity to calming signals and cranking up its sensitivity to excitatory ones to maintain balance.
When the alcohol leaves your system, those compensatory changes are still in place. Your brain is now in an unbalanced overdrive state: less responsive to calming signals, more responsive to stimulating ones. This is why hangovers come with anxiety, restlessness, a racing heart, and sensitivity to light and sound. That jittery, on-edge feeling the morning after isn’t psychological. It’s your nervous system literally running hotter than normal, with your stress response dialed up. The colloquial term “hangxiety” maps onto real neurobiology.
Alcohol Wrecks Your Sleep
You might pass out quickly after drinking, but the sleep you get is objectively poor. Alcohol reduces the time it takes to fall asleep and increases deep slow-wave sleep during the first half of the night. That sounds good until you learn what happens next. In the second half of the night, your brain hits a rebound effect: increased wakefulness, fragmented sleep, and suppression of REM sleep, the phase critical for emotional regulation, learning, and memory consolidation.
Higher doses make this worse. You’re more likely to wake up repeatedly, spend less total time actually asleep, and miss out on the restorative stages your brain needs. This is why you can sleep for eight or nine hours after drinking and still wake up feeling exhausted. The architecture of that sleep, the cycling between light, deep, and REM phases, has been scrambled. Much of the cognitive sluggishness and irritability of a hangover traces back to this disrupted sleep rather than alcohol’s direct chemical effects.
Dehydration and the Diuretic Effect
Alcohol suppresses a hormone called vasopressin that normally tells your kidneys to hold onto water. Without that signal, your kidneys let far more fluid pass through than usual. Early research estimated that every 10 grams of alcohol (roughly one standard drink) produces an extra 100 milliliters of urine output beyond what you’d normally produce. Over the course of an evening, that adds up fast.
This excess fluid loss contributes to the thirst, dry mouth, dizziness, and lightheadedness of a hangover. It also depletes electrolytes. But dehydration alone doesn’t explain the full picture. Studies have shown that rehydrating doesn’t resolve all hangover symptoms, which is why simply “drinking more water” helps but doesn’t cure things.
Your Stomach Gets Hit Directly
Alcohol irritates the lining of your stomach and increases acid production, particularly beverages like beer and wine. Research in the journal Gut found that beer and wine are potent stimulants of gastric acid secretion, with beer’s effect rivaling the maximum acid output your stomach can produce. Pure ethanol at high concentrations actually has a mild inhibitory effect on acid, meaning it’s other compounds in fermented drinks that really drive stomach irritation.
This acid surge, combined with direct irritation of the stomach lining, explains the nausea, abdominal pain, and occasional vomiting that come with a hangover. Alcohol also affects the muscles that control stomach emptying and movement through the intestines, which can lead to cramping and diarrhea. If you’ve ever noticed that beer and wine upset your stomach more than a clear spirit, the acid stimulation data backs that up.
Why Dark Liquors Make It Worse
Not all drinks produce equal hangovers, even at the same alcohol content. The difference comes down to congeners: complex organic molecules produced during fermentation and aging. These include compounds like acetone, tannins, and fusel oils. Darker spirits contain dramatically more of them. Bourbon has roughly 37 times the congener content of vodka.
A controlled study comparing bourbon and vodka at equivalent alcohol doses found that while both produced hangovers, people felt significantly worse after bourbon. The effect size for congener content on hangover severity was medium, meaning it’s a real and measurable contributor on top of the effects of alcohol itself. Clear spirits like vodka and gin won’t spare you a hangover entirely, but they do reduce one layer of the assault.
Why Some People Get Worse Hangovers
Genetics play a meaningful role. About 8% of the world’s population, roughly 540 million people, carry a variant of a gene involved in processing acetaldehyde, a toxic intermediate that forms when your liver breaks down alcohol. Among people of Asian descent, roughly 23.5% carry this variant, compared to less than 2.5% in other populations. Carriers accumulate more acetaldehyde in their blood after drinking, leading to facial flushing, headache, and heart palpitations on top of the usual hangover mechanisms. People with two copies of the variant (one from each parent) experience more severe symptoms than those with one.
Beyond genetics, other factors influence severity: how fast you drink, whether you ate beforehand, your body weight and composition, and how much sleep you got. Age matters too. The same amount of alcohol tends to produce worse hangovers as you get older, likely because your liver processes alcohol more slowly and your body recovers from inflammation less efficiently.
Why There’s No Single Cure
Because a hangover involves at least five or six distinct mechanisms happening simultaneously, no single remedy addresses all of them. Water helps with dehydration but doesn’t touch inflammation or nervous system rebound. Anti-inflammatory pain relievers can reduce headache and body aches but do nothing for disrupted sleep or stomach irritation (and can actually worsen gut symptoms). Food helps stabilize blood sugar but won’t calm an overstimulated nervous system.
The most effective approach is reducing the damage on the front end: drinking less, drinking slowly, eating beforehand, choosing lower-congener beverages, and alternating alcoholic drinks with water. Once a hangover is underway, you’re managing symptoms while your body works through several hours of recovery across multiple systems. Time remains the only intervention that resolves all of them.