A hangover is a collection of physical and mental symptoms that follow a bout of heavy drinking, typically hitting hardest the morning after. The most common symptoms include headache, nausea, fatigue, thirst, muscle aches, stomach pain, dizziness, sensitivity to light and sound, anxiety, irritability, and increased blood pressure. Your attention, decision-making, and coordination are also impaired during a hangover, even though the alcohol itself has largely left your system.
What Happens in Your Body
There’s no single cause of a hangover. Several overlapping processes kick in as your body breaks down alcohol, and together they produce that familiar misery.
When your liver processes alcohol, it first converts it into a toxic byproduct called acetaldehyde. Acetaldehyde is highly reactive and damaging to cells, and its buildup in your system is considered a major driver of hangover symptoms. Normally, a second enzyme quickly breaks acetaldehyde down into harmless acetic acid. But when you drink heavily, the system gets overwhelmed, and acetaldehyde lingers. The chemical reactions involved also generate harmful molecules called reactive oxygen species, which create oxidative stress in the liver and throughout the body.
On top of that, alcohol triggers a genuine inflammatory response. Blood levels of inflammatory markers rise significantly after heavy drinking, and the degree of that inflammation correlates directly with how bad the hangover feels. One study found that a protein called C-reactive protein, a standard marker of inflammation, rose by 50% the morning after drinking. Other inflammatory signals tracked closely with both the physical and psychological dimensions of hangover severity.
Dehydration and Blood Sugar Drops
Alcohol suppresses a hormone that tells your kidneys to conserve water. The result: drinking roughly four standard drinks causes your body to excrete 600 to 1,000 milliliters of extra water over the following hours. That’s up to a full quart of fluid lost beyond what you’d normally produce. Vomiting, sweating, and diarrhea during or after drinking can deepen that deficit. The thirst, dizziness, weakness, and dry mouth you feel during a hangover are classic signs of mild to moderate dehydration.
Alcohol also interferes with your liver’s ability to produce new glucose. Under normal circumstances, when your stored sugar reserves run low, your liver manufactures fresh glucose to keep your brain fueled. Alcohol metabolism shuts that process down. If you’ve been drinking without eating, your blood sugar can drop low enough to cause shakiness, weakness, nervousness, and a racing heart. These symptoms overlap with hangover symptoms so closely that many people don’t realize low blood sugar is part of what they’re experiencing.
Why You Feel Exhausted Despite Sleeping
Alcohol is a depressant, and it pushes your brain into deep sleep faster than usual during the first few hours of the night. That sounds beneficial, but it comes at a cost: REM sleep, the lighter, dream-rich stage critical for mental recovery, gets significantly reduced. Your brain spends more of the early night in deep sleep and less time in every other stage.
As alcohol wears off partway through the night, your nervous system rebounds. Sleep becomes fragmented, and you’re more likely to wake up or shift into lighter sleep stages. The total amount of REM sleep you get drops measurably. This disrupted sleep architecture is a major reason hangovers come with brain fog, poor concentration, and deep fatigue that feels disproportionate to how many hours you were technically asleep.
Why Some Drinks Feel Worse Than Others
Not all alcoholic drinks produce equal hangovers. The key difference is congeners, complex organic molecules produced during fermentation and aging. These include compounds like acetone, tannins, and additional acetaldehyde. Darker spirits carry far more of them. Bourbon contains roughly 37 times the congeners found in vodka.
In controlled studies, participants who drank bourbon reported significantly worse hangovers than those who drank the same amount of alcohol as vodka. The total alcohol consumed was identical; only the congener content differed. As a general rule, darker drinks like bourbon, red wine, and brandy tend to produce harsher hangovers than lighter options like vodka, gin, or white wine.
Genetics Play a Bigger Role Than You Think
How efficiently your body clears acetaldehyde is partly determined by your genes. A well-studied genetic variant affects the enzyme responsible for breaking down acetaldehyde. This variant is common among people of East Asian descent and is essentially absent in people of European or African ancestry.
People who carry two copies of this variant produce a nonfunctional version of the enzyme, meaning acetaldehyde builds up rapidly after even small amounts of alcohol. The result is an intense flushing reaction in the face, neck, and chest, along with nausea, headache, rapid heart rate, and drops in blood pressure. People with one copy of the variant experience a milder version of the same response. This genetic difference is a major reason why rates of heavy drinking and alcohol use disorder are significantly lower in East Asian populations.
Do Any Hangover Cures Actually Work?
The short answer: nothing has been proven to reliably cure a hangover. A systematic review examined every randomized, placebo-controlled trial on hangover treatments and found only very low quality evidence for any of them. No two studies even tested the same remedy, making it impossible to confirm results across experiments. A handful of substances showed modest improvements over placebo in individual small studies, including clove extract, red ginseng, and Korean pear juice, but none had strong enough evidence to earn a recommendation.
Water and electrolytes help address the dehydration component but won’t touch the inflammatory response or acetaldehyde buildup that drive much of the misery. Interestingly, coffee may actually make things worse. Research on enzyme activity found that coffee significantly reduced the body’s ability to break down acetaldehyde, potentially prolonging the hangover rather than easing it.
Pain relievers can take the edge off a headache, but they come with their own risks when your stomach lining is already irritated by alcohol. Eating before and during drinking slows alcohol absorption and helps maintain blood sugar, which can reduce severity. But once a hangover has fully set in, the only reliable cure remains time: your body needs hours to clear the remaining byproducts, resolve the inflammation, and restore normal fluid balance.