The Science of Hangovers: What Actually Causes Them and What Helps

Alex Harmon · Updated June 2026 · Reviewed for accuracy

You wake up with a pounding head, a queasy stomach, and a mouth that feels like a dry sponge. Everyone has experienced a hangover, but surprisingly few people understand what is actually happening inside their body. A hangover is not just "too much alcohol" — it is a cascade of overlapping physiological problems, each one making you feel worse in a different way. Understanding the real causes is the first step to dealing with them effectively.

Most hangovers peak around 6 to 8 hours after your last drink, often when your blood alcohol concentration (BAC) has already returned to zero. They typically resolve within 24 hours. The timing feels cruel, but it makes sense once you look at the biology.

The 5 Real Causes of a Hangover

1. Acetaldehyde Buildup

This is the main villain. When your liver breaks down ethanol, it first converts it into acetaldehyde — a toxic compound that is roughly 30 times more harmful than alcohol itself. Your liver then converts acetaldehyde into harmless acetate using an enzyme called ALDH2. The problem is that this second step is slower. Acetaldehyde accumulates, triggering inflammation, nausea, flushing, and that pounding headache. People with a genetic variant of ALDH2 (more common in East Asian populations) process acetaldehyde even more slowly, which is why they often experience intense flushing and discomfort after just one or two drinks.

2. Dehydration and Electrolyte Loss

Alcohol suppresses a hormone called vasopressin (also called antidiuretic hormone, or ADH), which normally tells your kidneys to reabsorb water. With vasopressin suppressed, your kidneys dump water into your bladder instead of keeping it in your body. For every standard drink, you lose roughly 100–150 ml more urine than you take in. Along with that water go electrolytes — sodium, potassium, and magnesium — that your muscles and nerves depend on. The result is classic dehydration: dry mouth, dizziness, fatigue, and a headache caused partly by reduced brain hydration.

3. Sleep Disruption

Alcohol makes you fall asleep faster, but it actively damages sleep quality. It suppresses REM sleep, the restorative stage where your brain processes memories and repairs itself. You may sleep 8 hours and wake up feeling like you barely slept. Alcohol also causes your body temperature to fluctuate, increases snoring, and causes you to wake more often in the second half of the night as the sedative effect wears off. Poor sleep amplifies every other hangover symptom — fatigue becomes exhaustion, mild headache becomes severe, and your ability to concentrate drops sharply.

4. Gastrointestinal Inflammation

Alcohol directly irritates the lining of your stomach and intestines. It increases the production of stomach acid and delays gastric emptying, which is why you feel nauseous and sometimes vomit. It also triggers an inflammatory response in the gut, releasing cytokines — signaling proteins that make you feel fatigued, achy, and generally unwell. This is the same mechanism behind the body aches you feel with the flu. Alcohol-induced inflammation in the GI tract is also why some people experience diarrhea the morning after drinking.

5. Low Blood Sugar (Hypoglycemia)

Alcohol interferes with your liver's ability to release glucose into the bloodstream. Your liver is busy processing alcohol, so it reduces gluconeogenesis — the process of creating glucose from stored glycogen. The result can be low blood sugar, which causes shakiness, weakness, difficulty concentrating, and irritability. This effect is worse if you were drinking on an empty stomach or skipped dinner entirely.

Why Darker Drinks Cause Worse Hangovers

Not all alcohol hits you the same way. Darker liquors consistently produce worse hangovers than lighter ones, and there is a well-established reason: congeners. Congeners are chemical byproducts of fermentation and aging — compounds like methanol, acetone, tannins, and various esters that give spirits their flavor and color. Methanol in particular is metabolized into formaldehyde and formic acid, both of which are toxic and significantly worsen hangover symptoms.

The general ranking from worst to mildest hangovers goes: whiskey and bourbon > dark rum > red wine > white wine > beer > vodka. A 2010 study published in Alcoholism: Clinical and Experimental Research confirmed that bourbon caused significantly worse hangovers than vodka at the same blood alcohol level. If you are prone to bad hangovers, switching to lighter-colored drinks like vodka or gin can make a genuine difference.

What Actually Helps

Rehydration (The Most Important Step)

Water helps, but plain water does not replace lost electrolytes. A better approach is to drink water alongside something with sodium and potassium. Pedialyte is popular for this reason — it was designed to rehydrate children with stomach illnesses and has the right balance of sodium, potassium, and glucose. Sports drinks like Gatorade work similarly, though they tend to be less concentrated in electrolytes. Drinking 500 ml of water before bed and keeping water nearby overnight makes a real difference to how you feel in the morning.

Food — But Timing Matters

Eating before or during drinking slows alcohol absorption and reduces peak BAC. A meal high in fat and protein is most effective at slowing absorption. The morning after, easily digestible foods help stabilize blood sugar: bananas provide potassium, toast raises blood sugar gently without irritating your stomach, and eggs provide cysteine, an amino acid that helps the liver break down acetaldehyde. However, eating greasy food after the fact is largely a myth — a burger at midnight does not "soak up" alcohol already in your bloodstream.

Pain Relief — Choose Carefully

For headache pain, ibuprofen (Advil, Nurofen) or aspirin are reasonable choices. They reduce inflammation and are safe to take with alcohol in your system. What you should absolutely avoid is acetaminophen (Tylenol, paracetamol). When alcohol is present in your liver — or has recently been processed — acetaminophen's metabolism produces a toxic compound called NAPQI. Your liver uses glutathione to neutralize NAPQI, but alcohol depletes glutathione reserves. The result can be serious liver damage, even at doses normally considered safe. This is not a rare edge case: alcohol and acetaminophen interactions send thousands of people to emergency rooms each year.

Sleep and Time

Rest genuinely helps. Sleep allows your body to repair itself, your liver to continue processing metabolites, and your immune system to reduce inflammation. There is no shortcut that replaces time — but proper rest makes the waiting more bearable.

What Does Not Work

• Hair of the dog: Drinking more alcohol temporarily suppresses withdrawal-like symptoms and may slightly delay the worst of the hangover, but it does not cure anything. It just pushes the problem forward — and risks building a dependency over time.
• Coffee: Caffeine is a diuretic. It may help you feel more alert temporarily, but it makes dehydration worse and can increase anxiety and heart rate, amplifying how rough you feel.
• Greasy food after drinking: Effective before drinking, not after. Once alcohol is absorbed into your bloodstream, food cannot remove it or speed up metabolism. Eating a large greasy meal after drinking may actually irritate an already inflamed stomach.
• Multivitamins and B vitamins: While alcohol does deplete B vitamins (particularly B1/thiamine), taking a vitamin supplement the morning after provides minimal hangover relief in practice. Clinical evidence for "hangover vitamins" is weak.

How to Prevent a Hangover

The most effective prevention strategies are also the most straightforward. Eat a substantial meal before you start drinking. Alternate alcoholic drinks with water — one glass of water between each drink makes a meaningful difference. Pace yourself to give your liver time to process alcohol (it can handle roughly one standard drink per hour). Choose lighter-colored drinks when possible. And stop before you exceed what your body can realistically handle overnight.