A hangover is your body’s reaction to drinking more alcohol than it can process efficiently, resulting in a cluster of symptoms that peak after your blood alcohol level drops back to zero. Most hangovers last about 12 hours from the time you wake up, though the full process from your last drink to feeling normal again averages around 18 hours. Roughly 23% of people appear to be resistant to hangovers, but for everyone else, the experience involves a predictable chain of biological events.
What Happens Inside Your Body
When you drink, your liver breaks alcohol down in two steps. First, an enzyme converts alcohol into a toxic byproduct called acetaldehyde. Then a second enzyme converts acetaldehyde into harmless acetate. The problem is that second step can’t always keep up, especially when you drink quickly or heavily. Acetaldehyde accumulates, and it’s significantly more toxic than alcohol itself.
That buildup does measurable damage at the cellular level. In animal studies, alcohol exposure reduced the ability of cells to produce energy by about 50% and slowed basic oxygen use by 30%. Acetaldehyde also generates a surge of unstable molecules that damage cells throughout the body, with particular effects on nerve tissue. This energy deficit and cellular stress are the foundation of nearly every hangover symptom you feel.
Your immune system responds to all this damage as though you’re fighting an infection. Levels of several immune signaling molecules rise significantly during a hangover, including ones involved in regulating inflammation. This immune activation is directly linked to the nausea, headache, fatigue, and diarrhea that define the hangover experience. Your body is essentially mounting an inflammatory response to the toxic aftermath of alcohol, which is why a hangover can feel surprisingly similar to coming down with a mild illness.
Why Your Stomach Feels Wrecked
Alcohol is a powerful stimulant of stomach acid production. Even at relatively low concentrations, it can push acid output to 58% to 82% of your stomach’s maximum capacity. This happens through several mechanisms, including direct stimulation of acid-producing cells and the release of histamine in the stomach lining. The result is an irritated, inflamed stomach lining that produces the nausea, queasiness, and sometimes vomiting or diarrhea that accompany a hangover. Over time, repeated exposure can lead to chronic inflammation of the stomach lining, though it doesn’t appear to cause ulcers on its own.
Blood Sugar and the Energy Crash
Your liver treats alcohol as a toxin and prioritizes breaking it down above its other jobs, including maintaining your blood sugar. Normally, when blood sugar starts to dip, the liver releases stored glucose to bring it back up. But while it’s busy processing alcohol, that safety net disappears. It takes roughly one hour for the liver to process a single standard drink, so six drinks means six hours of compromised blood sugar regulation. This is why hangovers often involve shakiness, weakness, mood changes, and difficulty concentrating. Your brain runs almost entirely on glucose, so even a modest dip hits your mental function hard.
The Role of Congeners
Not all drinks produce equal hangovers. Congeners are chemical byproducts of fermentation and distillation that give alcoholic beverages their distinctive flavors and colors. Dark spirits like bourbon and whiskey, along with red wine, contain significantly more congeners than clear spirits. Vodka is considered the “cleanest” option, with the fewest congeners and a reputation for producing milder hangovers at the same alcohol dose. Beer and wine also tend to have higher congener levels than distilled spirits. These compounds are thought to add their own toxic burden on top of the alcohol itself, making your liver work harder and worsening the overall hangover.
Timeline: When Symptoms Start and Stop
Hangover symptoms don’t begin while you’re still drunk. They start as your blood alcohol concentration approaches zero, which is why you typically wake up feeling terrible rather than going to bed feeling terrible. From your last drink, the full hangover arc averages about 18 hours. For most people, it falls somewhere between 14 and 23 hours total. From the time you wake up, expect about 12 hours before symptoms fade to near zero.
The pattern is not evenly distributed. Symptoms tend to be worst in the morning and decline more rapidly in the second half. In one study, severity scores dropped sharply starting around 16 hours after drinking, and by 21 hours most participants reported little to no remaining symptoms.
Why Some People Don’t Get Hangovers
About 23% of drinkers report never experiencing hangovers, a figure that holds remarkably consistent across different study designs and populations (ranging from 13% to 35% depending on the study). The reasons aren’t fully understood, but genetics play a clear role. Variations in the genes that code for alcohol-processing enzymes affect how quickly acetaldehyde builds up and how efficiently it gets cleared. One specific genetic variant common in some populations leads to noticeably more severe hangovers, suggesting that the speed and efficiency of your liver’s enzyme activity is a major factor in whether you’re susceptible.
Being hangover-resistant doesn’t mean alcohol isn’t doing damage. It simply means the inflammatory and metabolic disruptions either resolve faster or don’t cross the threshold that produces noticeable symptoms. The long-term health effects of heavy drinking are the same regardless of whether you feel rough the next morning.
What Actually Helps
Because a hangover involves dehydration, inflammation, low blood sugar, and stomach irritation simultaneously, no single remedy addresses everything. Drinking water between alcoholic drinks and before bed reduces the dehydration component. Eating before and during drinking slows alcohol absorption, giving your liver more time to keep up with acetaldehyde clearance. Choosing lower-congener drinks (clear spirits over dark ones) can reduce severity at the same alcohol dose.
Once a hangover has set in, time is the only reliable cure. Rehydrating with water or electrolyte drinks, eating bland carbohydrate-rich foods to restore blood sugar, and resting are the practical steps that support your body’s recovery process. Anti-inflammatory pain relievers can help with headache, though they add further irritation to an already inflamed stomach lining, so the timing and type matter.