A hangover is the collection of unpleasant physical and mental symptoms that follow a bout of heavy drinking, typically peaking once all the alcohol has left your bloodstream and lasting up to 24 hours or longer. It’s not a single problem but a cascade of overlapping effects: dehydration, inflammation, a toxic byproduct building up in your system, and your brain chemistry snapping back from the sedating effects of alcohol all at once.
What Causes a Hangover
There’s no single villain behind a hangover. Several mechanisms overlap, and researchers still debate which one matters most. But the major contributors are well established.
When your liver processes alcohol, it uses an enzyme to convert ethanol into a substance called acetaldehyde, a toxic intermediate that binds to proteins and other molecules in your body. A second enzyme then converts acetaldehyde into harmless acetate. The problem is that acetaldehyde, even in small amounts, causes a rapid pulse, sweating, nausea, vomiting, and skin flushing. If your liver can’t clear it fast enough, or if you’ve simply overwhelmed it with too much alcohol, the toxic effects of acetaldehyde linger into the next day even after your blood alcohol has returned to zero.
At the same time, alcohol suppresses your body’s production of vasopressin, the hormone that tells your kidneys to hold onto water. With less vasopressin circulating, your kidneys flush out far more fluid than you’re taking in, which is why you urinate so frequently while drinking. The resulting dehydration contributes to thirst, dizziness, and headache the next morning.
Your immune system also gets involved. Blood levels of several inflammatory signaling molecules, including IL-10, IL-12, and interferon-gamma, rise significantly during a hangover compared to normal conditions. This is essentially a low-grade inflammatory response, and it maps closely onto that general feeling of being unwell, the body aches, fatigue, and brain fog that make a hangover feel a bit like being sick.
Why Your Brain Feels Off
One of the more uncomfortable parts of a hangover is the anxiety, restlessness, and difficulty concentrating that can accompany it. This has a specific neurochemical explanation.
Your brain constantly balances two key chemical systems: one that calms neural activity and one that excites it. Alcohol tips the balance heavily toward the calming side, which is why drinking makes you feel relaxed and sedated. But your brain adapts quickly. To restore balance while you’re still drinking, it dials down its calming receptors and ramps up production of excitatory chemicals like glutamate.
Once the alcohol wears off, you’re left with a brain that has fewer calming receptors and an excess of excitatory activity. The result is a rebound effect: racing thoughts, heightened anxiety, irritability, and sensitivity to light and sound. This phenomenon is common enough that it’s been nicknamed “hangxiety.” For most people it resolves within a day, but it can be genuinely distressing, especially for anyone already prone to anxiety.
Common Symptoms and How Long They Last
Researchers have catalogued hangover symptoms systematically. The most commonly reported ones include fatigue, thirst, nausea, stomach pain, dizziness, headache, concentration problems, heart pounding, sweating, shivering, clumsiness, confusion, and apathy. Not everyone experiences all of these, and severity varies widely from person to person and from one drinking occasion to the next.
Symptoms typically begin as your blood alcohol concentration drops and peak right around the time it hits zero, usually several hours after your last drink. For most people, this means waking up feeling terrible. According to the National Institute on Alcohol Abuse and Alcoholism, symptoms can persist for 24 hours or longer, though the worst of it usually passes within 12 to 16 hours.
Why Some People Get Worse Hangovers
Genetics play a measurable role. A common gene variant affecting the enzyme that clears acetaldehyde is particularly prevalent in East Asian populations. People who carry one copy of this variant (known as inactive ALDH2) don’t process acetaldehyde efficiently. A study of Japanese workers found that these individuals experienced hangovers from significantly less alcohol than people with the fully active enzyme. The amount of drinking needed to trigger a hangover was markedly lower, and hangover frequency rose steeply with even moderate increases in daily consumption. People who flush red when drinking are often carrying this same variant, and flushing was independently associated with greater hangover susceptibility.
What you drink matters too, not just how much. Dark spirits like bourbon contain roughly 37 times more congeners than clear spirits like vodka. Congeners are complex byproducts of fermentation and aging, including substances like tannins, fusel oils, and methanol. A controlled study comparing bourbon and vodka at the same alcohol dose found that bourbon produced significantly worse hangovers, with a medium effect size for the difference. Interestingly, congeners didn’t affect next-day cognitive performance or sleep quality. They just made people feel worse.
Other individual factors include body weight, how quickly you drank, whether you ate beforehand, how well-hydrated you were, and how much sleep you got. Women generally reach higher blood alcohol levels than men from the same amount of alcohol relative to body weight, partly due to differences in body water content and enzyme activity.
What Actually Helps
There is no proven cure for a hangover. The supplement dihydromyricetin (DHM), widely marketed as a hangover remedy, has been studied in controlled settings. While early animal research suggested it might reduce intoxication symptoms, a human study found that DHM does not significantly speed up alcohol metabolism and does not decrease blood concentrations of either alcohol or acetaldehyde. It may even slow alcohol processing when alcohol is consumed repeatedly.
What does help is straightforward. Rehydrating with water or electrolyte drinks addresses the fluid loss caused by suppressed vasopressin. Eating bland, easy-to-digest food can settle your stomach and restore blood sugar, which drops during heavy drinking. Over-the-counter pain relief can take the edge off a headache, though anti-inflammatory options tend to work better than acetaminophen, which puts additional strain on a liver already busy processing alcohol byproducts. Sleep, when you can get it, gives your body time to clear the remaining inflammatory signals and restore neurochemical balance.
The only reliable way to prevent a hangover is to drink less. Pacing yourself, alternating alcoholic drinks with water, eating before and during drinking, and choosing lighter-colored beverages with fewer congeners all reduce severity. But once a hangover has set in, time is the only thing that truly resolves it.
The Broader Cost of Hangovers
Hangovers aren’t just a personal inconvenience. A 2019 analysis of the Dutch economy estimated that hangover-related productivity losses cost approximately €2.66 billion in a single year. The vast majority of that, over €2.4 billion, came not from people calling in sick but from “presenteeism,” showing up to work while hungover and performing poorly. That pattern likely holds across most economies: the biggest cost isn’t the missed day, it’s the impaired one.