What is a Hangover and what Causes a Hangover?

Hangovers are the set of unpleasant symptoms many people feel after consuming alcohol. They can include headache, thirst, nausea, fatigue, sensitivity to light and sound, poor concentration, and mood changes. Hangovers typically begin when blood alcohol concentration (BAC) falls toward zero and can range from a few hours to a day or more depending on how much was drunk and individual factors. 

How Long Does a Hangover Last

Duration of a hangover depends on the amount and type of alcohol consumed, drinking speed, hydration and food intake, sleep quality, genetics, sex, age, and overall health. Clinical reviews and consensus statements report that a typical alcohol hangover usually peaks when BAC returns to near zero and can last up to about 24 hours for many people, though in some cases symptoms may persist longer (some individuals report effects for 48–72 hours after heavy drinking). Expectable ranges therefore run from a few hours to one or more days depending on circumstances.

What Does a Hangover Feel Like?

Typical hangover symptoms include:
  • Extreme tiredness or weakness
  • Thirst and dry mouth (dehydration)
  • Headache and muscle aches
  • Nausea, vomiting, or stomach pain
  • Light and sound sensitivity
  • Trouble concentrating, brain fog, irritability, anxiety

What Causes a Hangover?

A hangover arises from multiple physiological processes triggered by alcohol. No single cause fully explains every symptom; instead, several mechanisms interact:

1. Dehydration and electrolyte imbalance

Alcohol is a diuretic - it increases urine production by inhibiting vasopressin (antidiuretic hormone) and can cause excess fluid and electrolyte loss. This fluid/electrolyte imbalance contributes to thirst, light-headedness, and headache. Replacing fluids and electrolytes helps relieve those particular symptoms.

2. Acetaldehyde buildup

When the liver processes ethanol it first converts it to acetaldehyde, a reactive, toxic intermediate. Although blood alcohol may be gone, acetaldehyde can persist and is strongly implicated in the nausea, flushing, and malaise associated with hangovers. Genetic differences in alcohol-metabolizing enzymes partly explain why people experience hangovers differently.

3. Nutrient depletion (B vitamins and others)

Alcohol consumption increases the body’s demand for certain nutrients and can reduce absorption or increase excretion of B-vitamins (B1, B2, B3, B6, B12, folate) and other micronutrients involved in normal energy metabolism. Lower levels of these cofactors may contribute to fatigue, low energy, and impaired cognitive function the next day.

4. Inflammation and oxidative stress

Alcohol triggers immune responses and raises inflammatory markers (cytokines, CRP) and oxidative stress in the body and brain. This neuro-inflammation is increasingly recognized as a central contributor to hangover severity - it correlates with worse headaches, mood changes, and cognitive impairment.

5. Gut irritation and increased intestinal permeability

Alcohol can disturb gut microbiota and increase intestinal permeability (“leaky gut”), allowing bacterial products to enter circulation and provoke immune responses. Gut irritation also contributes to nausea, stomach pain, and systemic inflammation linked to hangover symptoms.

Quick facts & figures

  • Many studies show a large proportion of social drinkers experience hangovers after heavy drinking episodes; incidence estimates vary by population and drinking patterns (some surveys find >60% incidence after heavy drinking episodes, while general-population surveys report lower annual rates). Individual susceptibility varies widely.

  • Hangovers can negatively affect work performance, driving ability, mood and safety; some economic analyses estimate substantial productivity costs at a national level