What Causes a Hangover? 5 Body Reactions Behind the Misery

A hangover is the set of symptoms that follows heavy drinking: headache, nausea, thirst, tiredness and a foggy head. No single cause explains it. Five processes overlap: a toxic breakdown product called acetaldehyde, fluid loss, inflammation, disrupted sleep and an irritated stomach and gut. Symptoms usually peak as the alcohol level in the blood falls toward zero, and they generally fade within about a day.

Why does breaking down alcohol make you feel sick?

The liver processes alcohol in two main steps. An enzyme called alcohol dehydrogenase converts ethanol, the alcohol in drinks, into acetaldehyde. A second enzyme, aldehyde dehydrogenase, then converts acetaldehyde into acetate, which the body can clear far more easily.

Acetaldehyde is toxic, and it is more harmful than the ethanol it comes from. While it lingers, it can contribute to nausea, sweating and a racing heart. The liver works through alcohol at a limited pace, on average roughly one standard drink per hour, so heavy drinking lets acetaldehyde and ethanol build up faster than they leave.

Why does alcohol leave you dehydrated?

Alcohol suppresses vasopressin, a hormone that tells the kidneys to hold on to water. With less of that signal, the kidneys send out more urine than the amount of liquid you drank would normally produce. The result is fluid loss along with a loss of salts such as sodium and potassium.

Thirst, dry mouth, dizziness and fatigue fit this picture. Dehydration is only part of a hangover, though. People who drink plenty of water during a night out can still feel awful the next day, which points to the other causes.

How do inflammation and stomach irritation add to it?

Alcohol irritates the lining of the stomach and increases stomach acid, which explains the nausea, stomach pain and vomiting many people report. It can also slow the emptying of the stomach.

The immune system takes part as well. Heavy drinking raises levels of inflammatory signaling molecules, the same chemical messengers the body uses when fighting an infection. That helps explain why a hangover can feel like a mild flu, with aches, poor concentration and low mood. Headaches have several possible triggers; a different and much shorter one is covered in the explainer on what causes brain freeze.

Alcohol can also push blood sugar down, which adds to weakness and shakiness in some people.

What does alcohol do to sleep?

Alcohol often makes people fall asleep quickly, but sleep later in the night is lighter and more broken. It cuts into REM sleep, the stage linked to vivid dreaming, and it can cause early waking. Anyone curious about that stage can read what researchers do and do not know in the article on why we dream.

A night of poor sleep alone causes tiredness, irritability and trouble concentrating, so it layers onto every other hangover symptom.

Why are some drinks and some people worse than others?

The amount of alcohol is the biggest factor. In the US, one standard drink holds the same amount of pure alcohol whether it is 12 oz (355 mL) of regular beer, 5 oz (148 mL) of wine or 1.5 oz (44 mL) of distilled spirits, according to the Centers for Disease Control and Prevention (CDC). Larger pours and stronger drinks quickly add up.

Other factors also play a part:

• Congeners, minor chemicals produced during fermentation and aging, are more plentiful in darker drinks such as bourbon and brandy than in vodka or gin.
• Body size, sex and genetics change how fast alcohol is processed and how high the blood level climbs.
• Drinking on an empty stomach speeds absorption, while food slows it down.
• Poor sleep, dehydration and illness before drinking can make symptoms worse.

What helps, and when is it an emergency?

Time is the only thing that clears a hangover. Rest, water and a light meal can ease thirst and nausea, but no drink, pill or food speeds up the liver. Coffee may make a person feel more alert and does not remove alcohol from the blood. Some over-the-counter pain relievers can irritate the stomach or strain the liver when combined with alcohol, so a pharmacist or doctor is the right person to ask.

Severe symptoms are not a hangover. The National Institute on Alcohol Abuse and Alcoholism (NIAAA) lists confusion, repeated vomiting, seizures, slow or irregular breathing, pale or bluish skin and unconsciousness as signs of alcohol poisoning. That needs emergency help right away (911 in the US, 999 in the UK). The CDC alcohol program publishes more information on alcohol and health.

Quick answers

How long does a hangover last?

Most last up to about 24 hours. Symptoms tend to be strongest once the blood alcohol level has dropped close to zero.

Does mixing drinks cause a worse hangover?

The total amount of alcohol matters more than the order or mix of drinks. Mixed drinks often contain more alcohol and sugar than people realize, which makes it easy to drink more than planned.

Can eating a big meal first prevent a hangover?

Food slows how fast alcohol enters the blood, but it does not stop the liver from producing acetaldehyde or alcohol from disrupting sleep. The only dependable way to avoid a hangover is to drink less or not at all.