A hangover is the result of several overlapping processes that start the moment your body begins breaking down alcohol. There’s no single cause. Instead, a toxic byproduct of alcohol metabolism, inflammation, dehydration, disrupted sleep, and irritation of your stomach lining all pile on at once. Symptoms typically hit full force the morning after heavy drinking, right around the time your blood alcohol level drops to zero.
Acetaldehyde: The Toxic Middleman
About 90% of the alcohol you drink gets processed in your liver through a two-step breakdown. First, enzymes convert alcohol into a compound called acetaldehyde, which is significantly more toxic than the alcohol itself. Then a second set of enzymes converts acetaldehyde into acetate, a relatively harmless substance your body can use for energy.
The problem is that second step can’t always keep up. When you drink faster than your liver can finish the job, acetaldehyde builds up in your bloodstream. This compound is directly responsible for many of the worst hangover feelings: nausea, sweating, rapid heartbeat, and a pounding headache. Reducing acetaldehyde levels faster is one of the few approaches that has shown real promise in easing hangover symptoms in clinical trials.
Your Immune System Fights Back
Drinking heavily triggers an immune response that resembles what happens when your body is fighting an infection. Researchers have found that levels of specific immune signaling molecules, particularly IL-10, IL-12, and interferon-gamma, rise significantly during a hangover compared to normal conditions. These are the same types of signals your body uses to coordinate inflammation when you’re sick, which explains why a hangover can feel so much like coming down with the flu. The fatigue, headache, nausea, and even diarrhea that come with a hangover are all linked to this disrupted immune signaling.
Dehydration and Electrolyte Loss
Alcohol suppresses a hormone that tells your kidneys to hold on to water. With that signal turned down, your kidneys let far more fluid pass through than they normally would, which is why you urinate so frequently while drinking. By the end of the night, you’ve lost substantially more water and electrolytes than you’ve taken in.
This fluid loss contributes to the thirst, dizziness, and lightheadedness you feel the next morning. It also concentrates sodium levels in your blood, which can worsen headaches. Replacing lost water won’t cure a hangover on its own, but sports drinks or other sources of electrolytes can take the edge off the dehydration component.
Your Liver Prioritizes Alcohol Over Blood Sugar
Your liver normally keeps your blood sugar stable by releasing stored glucose between meals and overnight. But when alcohol is in your system, the liver shifts its attention to detoxifying the alcohol and temporarily neglects its glucose-regulating duties. The result can be a drop in blood sugar that leaves you feeling shaky, weak, fatigued, and irritable the next morning. This is especially pronounced if you were drinking on an empty stomach or continued drinking late into the night without eating.
Alcohol Wrecks Your Sleep Quality
Even though alcohol can make you fall asleep quickly, the sleep you get is far worse than normal. Alcohol fragments your sleep cycle, causing your brain to briefly wake up over and over throughout the night. Each of these micro-awakenings can reset you to a lighter stage of sleep, cutting into the deep REM sleep your brain needs to restore itself. REM sleep is essential for feeling rested, and it plays a critical role in memory, mood regulation, and cognitive function.
This is why you can sleep eight hours or more after a night of drinking and still wake up feeling exhausted and foggy. The total hours of sleep matter less than the quality of the sleep you actually get, and alcohol degrades that quality significantly.
Congeners Make Some Drinks Worse Than Others
Not all alcoholic drinks produce equally bad hangovers, even at the same alcohol content. The difference comes down to congeners, which are chemical byproducts created during fermentation and aging. These include small amounts of methanol, tannins, and other compounds that give darker spirits their color and flavor.
Bourbon, whiskey, and red wine tend to be high in congeners. Vodka and other clear spirits contain far fewer. Studies dating back to the late 1950s have consistently shown that people report worse hangovers from darker spirits. In one controlled study, participants who drank bourbon had significantly higher hangover severity scores than those who drank the same amount of alcohol as vodka, with peak blood alcohol levels reaching about 0.11% in both groups. The congener content varied far more than the alcohol content across different whiskey brands tested, which helps explain why some bottles seem to hit harder than others.
That said, congeners appear to worsen how you feel without affecting cognitive performance any further. Mental sharpness and reaction time were similarly impaired regardless of which spirit people drank.
Genetics Play a Larger Role Than Most People Realize
If you’ve ever noticed that some people seem barely affected by the same amount of alcohol that leaves you miserable, genetics are a major reason. The enzymes responsible for breaking down alcohol and clearing acetaldehyde vary significantly from person to person based on inherited gene variants.
The most well-studied example involves a variant in the gene for the enzyme that clears acetaldehyde. Around 40 to 50% of people of East Asian descent carry a version of this gene that dramatically reduces the enzyme’s activity. In these individuals, even small amounts of alcohol can cause facial flushing, nausea, palpitations, and headaches, because acetaldehyde accumulates much faster and lingers longer. People who carry two copies of the variant (roughly 1 to 8% of East Asian populations) have the highest sensitivity and can experience intense reactions from very little alcohol. This variant is virtually absent in people of European descent, which accounts for some of the population-level differences in alcohol tolerance.
Other gene variants affect how quickly your body converts alcohol into acetaldehyde in the first place. Someone with a fast-acting version of this first enzyme paired with a slow-acting version of the second one gets hit with a double problem: acetaldehyde is produced rapidly but cleared slowly, creating a longer window of exposure to the toxic compound.
Stomach Irritation and Nausea
Alcohol is a direct irritant to the lining of your stomach. It increases acid production and inflames the stomach wall, which is why nausea, stomach pain, and sometimes vomiting are among the most common hangover symptoms. Carbonated alcoholic drinks can speed up alcohol absorption, and mixing alcohol with acidic mixers only adds to the irritation. Drinking on an empty stomach intensifies this effect because there’s no food to buffer the contact between alcohol and your stomach lining.
What Actually Helps (and What Doesn’t)
Because hangovers involve so many overlapping mechanisms, no single remedy addresses all of them. Drinking water or electrolyte-containing beverages helps with the dehydration piece but won’t touch the acetaldehyde buildup or immune activation. Eating before and during drinking slows alcohol absorption, giving your liver more time to keep up with the workload and reducing stomach irritation.
Choosing lower-congener drinks like vodka or gin over bourbon or whiskey can reduce symptom severity, though it won’t eliminate a hangover if you drink enough. Sleeping in a dark, quiet room helps, but alcohol’s disruption of your sleep architecture means the rest you get still won’t be fully restorative.
The only approach that reliably prevents all hangover mechanisms is drinking less. Your liver can process roughly one standard drink per hour. Staying closer to that pace, alternating alcoholic drinks with water, and stopping earlier in the evening gives your body the best chance of clearing acetaldehyde before it accumulates to levels that trigger the cascade of symptoms the next morning.