Hangovers are the result of several overlapping processes that unfold as your body breaks down alcohol. There isn’t one single cause. Instead, a combination of toxic byproducts, immune system activation, dehydration, blood sugar drops, and disrupted cellular energy production all contribute to the misery you feel the morning after drinking. Symptoms typically peak right around the time your blood alcohol level returns to zero, which is why you often feel worst several hours after your last drink rather than while you’re still buzzed.
Your Body Turns Alcohol Into a Toxin
When you drink, your liver processes alcohol in two steps. First, an enzyme converts ethanol into acetaldehyde, an intermediate chemical that is significantly more toxic than alcohol itself. A second enzyme then converts acetaldehyde into acetate, a relatively harmless substance your body can use for energy.
The problem is that first step often outpaces the second. When acetaldehyde accumulates faster than your body can clear it, you experience a rapid pulse, sweating, skin flushing, nausea, and vomiting. Some people carry a genetic variant of the enzyme responsible for that second step, which makes it slower or less effective. These individuals flush, sweat, and feel sick after even small amounts of alcohol because acetaldehyde builds up quickly and lingers longer. This variant is particularly common in people of East Asian descent.
Alcohol Triggers an Immune Response
Heavy drinking provokes your immune system in ways that mirror what happens when you’re fighting an infection. After a night of drinking, your body ramps up production of signaling molecules called cytokines, the same chemicals responsible for the aches, fatigue, and foggy thinking you experience with the flu. Studies measuring blood and saliva after alcohol consumption have found significant elevations in several of these molecules, and the size of the increase correlates with how severe the hangover feels.
Specifically, higher levels of certain inflammatory markers are linked to worse headaches and concentration problems the next day. Others correlate with physical symptoms like body aches and general malaise. This is why a hangover can feel so much like being sick: your immune system is genuinely activated, producing the same chemicals that make infections feel awful.
Dehydration Is Real, but Not the Whole Story
Alcohol suppresses a hormone called vasopressin, which normally tells your kidneys to hold onto water. When vasopressin drops during drinking, your kidneys let more fluid pass through, which is why you urinate so frequently after a few drinks. Your vasopressin levels typically drop while you’re drinking and then rebound after you stop, but by that point you’ve already lost a significant amount of fluid.
This fluid loss contributes to thirst, dry mouth, and dizziness. But dehydration alone doesn’t fully explain hangovers. Studies have shown that rehydrating doesn’t reliably eliminate hangover symptoms, which means the other mechanisms, particularly acetaldehyde toxicity and inflammation, are doing most of the heavy lifting. Dehydration makes things worse, but it’s a supporting player rather than the main villain.
Your Blood Sugar Drops
Alcohol interferes with your liver’s ability to produce new glucose, a process called gluconeogenesis. It can also suppress the release of stored glucose and may increase insulin secretion, all of which push your blood sugar lower than normal. This combination creates a mild form of low blood sugar that contributes to the fatigue, irritability, weakness, and shakiness many people feel during a hangover.
Your brain runs almost entirely on glucose, so even a modest dip affects your ability to think clearly and sustain energy. This is one reason eating a substantial meal before or during drinking can soften hangover severity: it helps maintain a more stable blood sugar baseline and slows alcohol absorption.
Your Cells Struggle to Produce Energy
Research in animal models has shown that hangovers involve measurable damage to mitochondria, the structures inside your cells that generate energy. During a hangover state, mitochondria in brain tissue showed a 55% reduction in their ability to efficiently produce energy, along with decreased activity in key parts of their energy-production machinery. At the same time, production of damaging molecules called free radicals jumped sharply: superoxide (a reactive oxygen species) increased by 25%, and hydrogen peroxide surged by 92%.
This combination of impaired energy production and increased oxidative stress helps explain the profound exhaustion that characterizes a bad hangover. Your cells are literally less capable of generating the energy your brain and body need, while simultaneously dealing with a flood of molecules that damage cellular components.
Dark Liquors Make It Worse
Not all alcoholic drinks produce equally bad hangovers, and the difference comes down to congeners. These are minor chemical compounds produced during fermentation and distillation, including substances like methanol, tannins, and various aldehydes. Darker spirits like bourbon, whiskey, and red wine contain far more congeners than clear spirits like vodka or gin.
Experimental studies confirm that bourbon produces more severe hangover ratings than vodka at the same level of intoxication. That said, the ethanol itself has a considerably stronger effect on hangover severity than congener content does. Switching to vodka won’t prevent a hangover if you drink enough of it. But at the margins, choosing lower-congener drinks can modestly reduce how rough the next morning feels.
Genetics Influence Your Susceptibility
How badly you experience hangovers is partly inherited. The genes that code for alcohol-processing enzymes vary from person to person, and these variations meaningfully affect how quickly you accumulate and clear acetaldehyde. People with gene variants associated with the alcohol flush response consistently report more severe hangovers, even at lower levels of consumption.
This genetic component also explains why some people seem nearly immune to hangovers while others suffer intensely after just a few drinks. It’s not simply a matter of tolerance or hydration habits. The speed and efficiency of your personal enzyme machinery plays a significant role in determining where you fall on the hangover spectrum.
Why Symptoms Hit After You Stop Drinking
One of the counterintuitive things about hangovers is that you feel worst after the alcohol has left your system, not while it’s still present. Alcohol has mild pain-dulling and mood-elevating effects while it’s active in your blood. As your blood alcohol level drops toward zero, those masking effects fade and the accumulated damage becomes apparent: inflammation is peaking, acetaldehyde metabolites are still circulating, your fluid balance is disrupted, blood sugar is low, and mitochondria are struggling.
This is also why “hair of the dog” (drinking more alcohol to relieve a hangover) provides temporary relief. It reintroduces the sedative and analgesic effects of alcohol, delaying the full reckoning. It doesn’t resolve any of the underlying processes and simply postpones them.