A hangover is the collection of negative mental and physical symptoms you experience after a night of drinking, starting when your blood alcohol level drops back toward zero. It’s not the drunkenness itself but what comes after: headache, nausea, fatigue, and a general feeling that your body is working hard to recover from something toxic. Symptoms can last 24 hours or longer.
Why Hangovers Start When You’re “Sober”
This is the counterintuitive part. You don’t feel hungover while you’re still drunk. Hangover symptoms peak right around the point when your blood alcohol concentration hits zero. That’s because much of what makes you feel terrible isn’t the alcohol itself but what your body does while processing it and the aftereffects once it’s gone.
Your liver breaks down alcohol in two steps. First, it converts ethanol into a substance called acetaldehyde, which is highly reactive and toxic. Then a second enzyme converts acetaldehyde into acetate, which is harmless. The problem is that acetaldehyde, even at moderate concentrations, causes a rapid pulse, sweating, flushing, nausea, and vomiting. Although acetaldehyde clears from your blood by the time your BAC reaches zero, its toxic effects on your tissues can linger well into the next day.
What’s Actually Happening in Your Body
A hangover isn’t one problem. It’s several overlapping ones hitting you at the same time.
Inflammation
Drinking triggers an immune response that looks surprisingly similar to fighting off an infection. Levels of specific immune signaling molecules (called cytokines) rise significantly during the hangover state compared to normal conditions. This inflammatory response is thought to drive the nausea, headache, diarrhea, and fatigue that feel so familiar the morning after. It’s the same basic mechanism that makes you feel achy and wiped out when you have the flu.
Dehydration and Fluid Imbalance
Alcohol suppresses a hormone called vasopressin that tells your kidneys to hold onto water. Without it, your kidneys let far more fluid pass through than they normally would, which is why you urinate so frequently while drinking. The result is dehydration that contributes to headache, dry mouth, dizziness, and thirst. This fluid loss also depletes electrolytes your muscles and nerves need to function properly.
Headache
Hangover headache is remarkably common. In a large survey of Danish adults, 72 percent reported experiencing it at some point, making it the most frequently reported type of headache. Alcohol causes blood vessels to widen, which may contribute directly. It also affects hormones and chemical messengers involved in pain signaling, including histamine, serotonin, and prostaglandins. Despite how universal hangover headaches are, researchers still don’t fully understand the exact cause.
Disrupted Sleep
Even if you pass out quickly after drinking, the quality of sleep you get is poor. Alcohol suppresses deep, restorative sleep (particularly REM sleep) during the first half of the night. Then during the second half, your brain overcompensates with a “rebound” of REM sleep and increased wakefulness. The net effect is that you wake up more often, sleep less deeply, and feel exhausted even after what seemed like a full night in bed. This disrupted sleep is a major reason hangovers leave you feeling so drained.
Anxiety and Jitteriness
While you’re drinking, alcohol enhances calming brain activity and suppresses stimulating activity. Once alcohol leaves your system, your brain swings in the opposite direction. Stimulating neurotransmitter systems rebound, which can leave you feeling anxious, on edge, or shaky the next morning. This is sometimes called “hangxiety,” and it’s especially noticeable in people who are already prone to anxiety.
Why Some Drinks Cause Worse Hangovers
Not all alcohol is created equal when it comes to the morning after. Alcoholic beverages contain small amounts of chemical byproducts from fermentation called congeners. These include substances like acetone, tannins, and methanol, and they add to the toxic load your body has to process.
Dark spirits carry far more congeners than clear ones. Bourbon contains roughly 37 times the amount of congeners found in vodka. In controlled studies, people drinking bourbon reported significantly worse hangovers than those drinking vodka, even when the actual alcohol content was the same. The breakdown of methanol, in particular, produces formaldehyde and formic acid as byproducts, and its elimination from the body coincides with the onset of hangover symptoms. So choosing lighter-colored drinks won’t prevent a hangover, but it may reduce how severe it feels.
How Long a Hangover Lasts
Most hangovers follow a predictable arc. Symptoms peak as your blood alcohol level returns to zero, which for a night of heavy drinking is typically the morning after. From there, symptoms gradually ease but can persist for a full 24 hours or longer depending on how much you drank, your body size, and individual factors like genetics and overall health.
There is no shortcut. Recovery requires your body to finish clearing the toxic byproducts of alcohol metabolism, rehydrate, heal irritated tissue in the stomach and intestines, and restore normal immune and brain function. That simply takes time.
What Actually Helps (and What Doesn’t)
Despite the enormous market for hangover “cures,” no treatment has demonstrated effectiveness in independent, double-blind, placebo-controlled clinical trials. That includes popular remedies like activated charcoal, vitamin supplements, and various pills marketed specifically for hangovers. None have cleared the bar of rigorous scientific testing.
What does have some support is more modest. People whose diets are higher in zinc and a B vitamin called nicotinic acid tend to report less severe hangovers. Staying hydrated while drinking (alternating alcoholic drinks with water) helps offset fluid loss, though it won’t eliminate a hangover entirely. Eating before and during drinking slows alcohol absorption, which reduces the peak concentration of acetaldehyde your body has to manage.
Beyond that, the most effective strategies are the boring ones: drink less, choose lower-congener beverages, pace yourself, hydrate, eat beforehand, and give your body the time it needs to recover.