A hangover is the result of several biological processes happening at once, not a single cause. When you drink alcohol, your body breaks it down into toxic byproducts, triggers an immune response, suppresses key hormones, disrupts your sleep architecture, and irritates your stomach lining. These effects overlap and compound each other, which is why a hangover can feel like being sick, sleep-deprived, and anxious all at the same time.
How Your Body Processes Alcohol
Your liver breaks down alcohol in two steps. First, an enzyme converts alcohol into acetaldehyde, a toxic and highly reactive compound. Then a second enzyme converts acetaldehyde into acetate, which is harmless. In most people, that second step happens quickly enough that acetaldehyde doesn’t build up to dangerous levels. But even brief exposure to it during a night of drinking causes real damage. At higher concentrations, acetaldehyde binds to proteins and other important molecules in your cells, triggering a rapid pulse, sweating, skin flushing, nausea, and vomiting.
By the time your blood alcohol level returns to zero, free acetaldehyde is no longer circulating. But the toxic effects it caused while your body was processing alcohol persist into the hangover period. Think of it like a bruise: the impact is over, but the damage lingers. Hangover symptoms actually peak right around the time your blood alcohol concentration hits zero, which for a heavy night of drinking is typically the next morning.
Your Immune System Fires Up
Drinking heavily activates parts of your immune system in ways that resemble fighting an infection. Researchers have found that during a hangover, blood levels of several immune signaling molecules (specifically IL-10, IL-12, and interferon-gamma) rise significantly compared to normal conditions. These are the same types of molecules your body releases when you have the flu, and they produce that familiar feeling of general malaise, fatigue, and achiness. This is why a hangover can feel so much like being sick: your immune system is genuinely inflamed.
Dehydration and Fluid Loss
Alcohol suppresses vasopressin, the hormone that tells your kidneys to hold onto water. When vasopressin drops during drinking, your kidneys let far more fluid pass through than they normally would. That’s why you urinate so frequently while drinking. The result is a net loss of fluids and electrolytes that contributes to the headache, dry mouth, dizziness, and thirst you feel the next day.
Vasopressin levels typically rebound after you stop drinking, sometimes overshooting to higher-than-normal levels. But by then the damage is done. The fluid deficit you built up over several hours of drinking isn’t immediately corrected, and your body spends much of the hangover period trying to restore balance.
Disrupted Sleep
Even if you sleep for eight hours after drinking, the quality of that sleep is poor. Alcohol changes your sleep architecture in a specific, two-phase pattern. In the first half of the night, when your blood alcohol is still high, you fall asleep faster and spend more time in deep sleep. This sounds beneficial, but it comes at a cost: your brain suppresses REM sleep, the phase critical for memory consolidation and emotional regulation.
In the second half of the night, as your body finishes processing the alcohol, sleep falls apart. You experience more wakefulness, more time in the lightest stage of sleep, and frequent disruptions. The net result is that you wake up having missed out on the most restorative phases of sleep. This contributes directly to the brain fog, irritability, and difficulty concentrating that define a hangover morning.
The Glutamate Rebound and “Hangxiety”
While you’re drinking, alcohol enhances the effects of your brain’s main calming chemical (GABA) and suppresses glutamate, an excitatory chemical. That’s why alcohol makes you feel relaxed and slows your reaction time. But your brain adapts to this suppression in real time. When the alcohol wears off, glutamate activity rebounds and overshoots its normal level, leaving your nervous system in a hyper-excitable state.
This rebound is responsible for the anxiety, restlessness, and light sensitivity many people experience during a hangover, sometimes called “hangxiety.” Research in animal models has shown that this glutamate surge, particularly in brain regions that regulate fear and anxiety, plays a significant role in hangover-related nervousness. If you already tend toward anxiety, this effect can be especially pronounced.
Stomach Irritation
Alcohol irritates your stomach lining directly, but the type of drink matters more than you might expect. Beer and wine are powerful stimulants of stomach acid production, with beer triggering acid output equal to the maximum your stomach can produce. Higher-proof spirits like whisky, gin, and cognac do not stimulate acid secretion in the same way. Pure ethanol at low concentrations (below about 5%) mildly stimulates acid, while higher concentrations have little effect or even slightly inhibit it.
This means the nausea and stomach discomfort of a hangover aren’t purely about how much alcohol you consumed. The non-alcohol compounds in beer and wine, including fermentation byproducts, play a significant role in triggering the gastric distress you feel the next day.
Why Dark Liquors Make It Worse
Alcoholic beverages contain small amounts of biologically active compounds called congeners, which are byproducts of fermentation and aging. Dark spirits like bourbon contain far more congeners than clear spirits like vodka. In controlled studies, bourbon produced notably more severe hangover ratings than vodka at the same alcohol dose. However, the alcohol itself still had a considerably stronger effect on hangover severity than congener content alone. Choosing clear spirits over dark ones may take the edge off, but it won’t prevent a hangover if you drink enough.
Genetics Play a Real Role
Some people are genetically wired to have worse hangovers. The most well-studied example involves variations in the gene for aldehyde dehydrogenase, the enzyme responsible for clearing acetaldehyde from your system. People who carry less efficient versions of this enzyme metabolize acetaldehyde slowly, allowing it to accumulate even after small amounts of alcohol. The result is intense flushing, nausea, rapid heartbeat, and low blood pressure.
These genetic variants are most common among people of East Asian ancestry, though they occur in other populations too. The visible sign is a red face or neck during or after drinking, sometimes called the “alcohol flush reaction.” People with this variation aren’t just experiencing a cosmetic reaction. They’re accumulating a toxic compound at levels that make every drink hit harder and every hangover feel worse. The flush is essentially a warning that acetaldehyde is building up faster than the body can clear it.
Why Hangovers Get Worse With Age
Several of the mechanisms behind hangovers become less efficient as you age. Your liver processes alcohol more slowly, your body holds less water (making dehydration hit harder), your sleep quality is already declining, and your immune system becomes more reactive to inflammatory triggers. None of these changes are dramatic year to year, but cumulatively they explain why the same amount of alcohol that barely fazed you at 22 can flatten you at 35. Your body is running the same set of recovery processes with fewer resources.