A hangover isn’t caused by any single thing. It’s the result of several overlapping processes: your body breaking down a toxic byproduct of alcohol, an inflammatory immune response, disrupted sleep, irritation of your stomach lining, and shifts in fluid balance and blood sugar. These mechanisms hit at different times and interact with each other, which is why hangovers feel like such a full-body experience. Symptoms peak when your blood alcohol level drops back to zero and can last 24 hours or longer.
Your Body Produces a Toxic Byproduct
When you drink, your liver breaks down alcohol in two steps. First, it converts ethanol into a substance called acetaldehyde. Then a second enzyme converts acetaldehyde into acetate, which is harmless. The problem is that middle step. Acetaldehyde is chemically reactive and toxic. At higher concentrations, it causes a rapid pulse, sweating, skin flushing, nausea, and vomiting.
In most people, the second enzyme works fast enough that acetaldehyde doesn’t pile up too much. But when you drink heavily, the system gets overwhelmed. And even after your blood alcohol reaches zero, the toxic effects of acetaldehyde produced during metabolism can persist into the hangover period. Your body has already cleared the alcohol, but the damage from processing it lingers.
Your Immune System Treats It Like an Infection
One of the more surprising causes of hangover misery is inflammation. Alcohol triggers your immune system to release the same signaling molecules it uses to fight illness. Blood levels of inflammatory markers, particularly ones involved in fever and pain signaling, rise significantly after drinking and correlate directly with how bad the hangover feels the next day.
The connection is strong enough that researchers have found the level of C-reactive protein (a general marker of inflammation) measured just four hours after drinking can predict hangover severity 10 hours later. This inflammatory cascade explains a lot of what people describe as feeling “sick” during a hangover: the body aches, the headache, the general sense that something is wrong. Your immune system is responding to the chemical stress of processing alcohol, including the reactive oxygen species produced when your liver converts ethanol to acetaldehyde. Some of these byproducts bind to proteins in your body and get flagged as foreign invaders, prompting your immune system to ramp up even further.
Alcohol Disrupts Your Fluid Balance
Alcohol suppresses the hormone that tells your kidneys to hold onto water. Normally, this hormone (called vasopressin or antidiuretic hormone) keeps you from losing too much fluid. During drinking, your blood levels of vasopressin drop, which is why you urinate far more than the volume of liquid you’re taking in. When you stop drinking, vasopressin levels rebound upward, but by then you’ve already lost a significant amount of fluid and electrolytes.
This doesn’t fully explain hangovers on its own. Studies have found that dehydration alone doesn’t account for all hangover symptoms. But it contributes to the thirst, dry mouth, dizziness, and lightheadedness that most people experience the morning after.
Your Stomach Takes a Hit
Alcohol irritates the lining of your digestive tract, but not all drinks do it the same way. Lower-alcohol beverages like beer and wine are actually stronger stimulants of stomach acid production than hard liquor. Beer stimulates acid output at levels equal to the stomach’s maximum capacity. Whisky, gin, and cognac, despite their higher alcohol content, do not trigger the same acid surge.
This is partly why beer and wine can leave some people feeling especially nauseated the next morning, even at the same total alcohol intake. The excess acid irritates the stomach lining and contributes to the nausea, abdominal pain, and general queasiness that characterize a hangover.
Blood Sugar Drops While You Sleep
Your liver normally produces glucose between meals and overnight to keep your blood sugar stable. Alcohol disrupts this process. The byproducts of alcohol metabolism block the liver from making new glucose, causing blood sugar to fall. For most healthy people, this results in fatigue, shakiness, weakness, and mood changes the next morning. For people with diabetes, the drop can be more dangerous.
Sleep Gets Worse, Not Better
Alcohol might help you fall asleep faster, but it wrecks the quality of that sleep. It reduces sleep efficiency, cuts into REM sleep (the deep, restorative phase tied to memory and mental recovery), and increases the number of times you wake during the night. People with more disrupted sleep after drinking report worse hangovers, and hangover severity correlates with poorer cognitive and psychomotor performance the next day.
This is why you can sleep eight hours after drinking and still wake up exhausted. The architecture of the sleep itself is fragmented. Less time in REM sleep means your brain doesn’t get the recovery it needs, which compounds the fatigue, brain fog, and difficulty concentrating that come from all the other hangover mechanisms at work.
Dark Liquors Make It Worse
Not all alcoholic drinks produce equal hangovers. A key factor is congeners: complex organic molecules produced during fermentation and aging. These include compounds like tannins, acetone, and fusel oils, all of which have their own toxic effects. Bourbon contains roughly 37 times the amount of congeners as vodka.
In controlled studies where participants drank enough bourbon or vodka to reach the same blood alcohol level, hangover ratings were consistently higher after bourbon. The main driver of any hangover is still the ethanol itself, but congeners measurably amplify symptom severity. This is why darker spirits like bourbon, brandy, and red wine tend to produce rougher mornings than clear spirits like vodka or gin.
Genetics Play a Real Role
Some people are genetically wired to have worse reactions to alcohol. The most well-studied example involves a variant of the ALDH2 gene, which codes for the enzyme responsible for clearing acetaldehyde from your body. People who carry a less efficient version of this gene can’t break down acetaldehyde as quickly, so it accumulates even after small amounts of alcohol. The result is immediate flushing, nausea, rapid heartbeat, and sweating.
This variant is most common in East Asian populations and is rare outside that group. It explains a significant portion of why some people get violently ill from a single drink while others can consume much more before feeling effects. The variant accounts for about 29% of the variation in flushing response in studied populations. If you’ve always wondered why hangovers hit you harder than your friends, your enzyme genetics are likely part of the answer.
Why It All Hits at Once
The reason hangovers feel so terrible is that none of these mechanisms operate in isolation. Your immune system is inflamed while your stomach lining is irritated. Your blood sugar is low while your sleep has been fragmented. You’re dehydrated while your body is still dealing with the residual toxic effects of acetaldehyde. Each system compounds the others, and they all converge right around the time your blood alcohol hits zero, typically the morning after a night of heavy drinking. The more you drink, the more each of these pathways gets activated, and the worse the combined effect.