Hangovers are the result of several overlapping problems happening in your body at once. There’s no single cause. Alcohol triggers a chain reaction that affects your liver, stomach, brain chemistry, immune system, and sleep quality, and the combined fallout is what you feel the next morning.
How Your Body Breaks Down Alcohol
When you drink, your liver converts ethanol into a compound called acetaldehyde, which is significantly more toxic than alcohol itself. Acetaldehyde is eventually broken down further into harmless acetic acid (essentially vinegar) and cleared from your body. But that middle step, where acetaldehyde is circulating in your system, is where much of the damage happens. It irritates tissues and contributes directly to hangover symptoms like headache, nausea, and a racing heart.
How fast you clear acetaldehyde depends heavily on your genetics. About 8% of the global population, primarily people of East Asian descent, carry a mutation in the enzyme responsible for breaking down acetaldehyde. Roughly 45% of Han Chinese individuals have this variant, which causes acetaldehyde to build up in the bloodstream instead of being processed efficiently. That’s why some people experience intense facial flushing, nausea, and rapid heartbeat after even small amounts of alcohol. If hangovers hit you harder than your friends, your enzyme activity may simply be slower.
Your Brain’s Chemical Overcorrection
Alcohol doesn’t just sedate you randomly. It works by boosting the activity of your brain’s calming system (GABA) while simultaneously suppressing the excitatory signals (glutamate) that normally keep you alert and on edge. That’s the relaxed, uninhibited feeling of being buzzed.
The problem comes when alcohol wears off. Your brain doesn’t gently return to baseline. It overcorrects, dialing GABA way down and cranking glutamate back up. The result is the opposite of what you felt while drinking: restlessness, anxiety, irritability, and that jittery, wired-but-exhausted feeling the morning after. This rebound effect is the main reason some people experience intense next-day anxiety, sometimes called “hangxiety.” It’s not psychological. It’s a measurable chemical imbalance in your brain that resolves as your neurotransmitters rebalance over the course of the day.
Why Hangover Sleep Doesn’t Feel Like Real Sleep
Alcohol initially acts like a sedative, which is why you might fall asleep quickly after drinking. But it disrupts your sleep architecture in a specific and consistent way. During the first half of the night, alcohol suppresses REM sleep, the stage most closely linked to memory processing and mental restoration. During the second half, as your blood alcohol drops, your brain experiences a rebound: more frequent wake-ups, more transitions between sleep stages, and fragmented rest overall.
So even if you were technically in bed for eight hours, the quality of that sleep was poor. Your brain spent less time in the restorative stages it needed. This is why hangovers come with brain fog, difficulty concentrating, and that heavy, unrested feeling that a normal night of short sleep doesn’t quite produce.
Low Blood Sugar and the Liver Bottleneck
Your liver has a limited capacity, and when it’s busy metabolizing alcohol, other jobs get sidelined. One of those jobs is producing glucose. Normally, your liver maintains your blood sugar between meals through a process called gluconeogenesis. Alcohol directly interferes with this process by changing the chemical environment inside liver cells, blocking the conversion of key precursors your liver needs to manufacture glucose.
The result can be a meaningful drop in blood sugar, especially if you were drinking on an empty stomach or didn’t eat before bed. Low blood sugar contributes to the shakiness, weakness, fatigue, and difficulty thinking clearly that characterize a bad hangover. In extreme cases of binge drinking, this suppression of glucose production can cause dangerously low blood sugar levels.
What Alcohol Does to Your Stomach
Even at relatively low concentrations, alcohol stimulates your stomach to produce more acid. This happens through several pathways, including direct stimulation of the acid-producing cells in your stomach lining and triggering the release of histamine, which further ramps up acid output. Certain compounds naturally present in alcoholic beverages, like succinic and maleic acid, add to the effect.
This increased acid production, combined with alcohol’s direct irritation of the stomach lining, explains the nausea, stomach pain, and sometimes vomiting that come with a hangover. Your digestive system is essentially inflamed and overstimulated, and it takes time for the lining to calm down.
Dehydration Plays a Supporting Role
Alcohol is a diuretic, meaning it causes your kidneys to release more water than you’re taking in. This is why you urinate more frequently when drinking. The resulting dehydration contributes to headache, dry mouth, dizziness, and fatigue. It’s a real factor, but it’s not the whole story. Studies have found that rehydrating alone doesn’t fully resolve hangover symptoms, which is why “just drink water” has never been a complete cure.
Why Some Drinks Cause Worse Hangovers
Not all alcoholic drinks are created equal when it comes to hangovers. The key variable is congeners: byproducts of fermentation and distilling that give drinks their color, flavor, and aroma. With the same amount of alcohol consumed, darker spirits like bourbon, brandy, and whiskey tend to produce worse hangovers than lighter options like vodka or beer.
Methanol is one of the most studied congeners. It’s found in the highest concentrations in red wine, brandy, and whiskeys, and in the lowest concentrations in beer and vodka. Your body breaks methanol down into formaldehyde and formic acid, both of which are highly toxic. Interestingly though, when researchers have tried to directly correlate urine methanol levels with overall hangover severity, the relationship hasn’t been strong. The one exception was vomiting, which did correlate significantly with methanol levels. So congeners likely play a real but partial role, layered on top of all the other mechanisms.
Why Some People Get Worse Hangovers Than Others
Beyond the genetic enzyme differences already mentioned, several factors influence how severe your hangover will be. Body weight and composition affect how concentrated alcohol becomes in your blood. Women generally reach higher blood alcohol levels than men at the same dose due to differences in body water content and enzyme activity. How quickly you drink matters enormously, since your liver can only process roughly one standard drink per hour. Drinking on an empty stomach accelerates absorption and worsens every downstream effect.
Age also plays a role, though not in the direction most people assume. Your liver’s efficiency at processing alcohol and its byproducts can decline with age, meaning the same amount of drinking that barely affected you at 22 may produce a two-day hangover at 35. Your body’s overall recovery capacity, hydration baseline, and sleep quality all shift over time, compounding the effect.