A hangover is not one thing happening to you. It’s at least five things happening to you at once, all caused by the way your body processes alcohol. Once you understand each one, the whole experience makes sense, and so do the reasons that some interventions work and most don’t. This article is part of our Hangovers hub, the complete guide to preventing and recovering from a hangover.

This article goes deep on the physiology. If you only want practical advice, the pillar covers the essentials. This is for when you want to know why.

# How alcohol enters and leaves your body

Before the hangover, the journey of an alcohol molecule:

Absorption. Alcohol enters your bloodstream primarily through the small intestine, with smaller amounts absorbed through the stomach lining. Carbonated drinks (champagne, beer, hard seltzers) absorb faster than still ones. Food in your stomach slows absorption substantially. On an empty stomach, alcohol peaks in your blood within 30-45 minutes; with food, peak is delayed by an hour or more.

Distribution. Once in the bloodstream, alcohol crosses freely into water-rich tissues, including your brain. This is why brain effects (relaxation, slurred speech, impaired judgment) follow your blood alcohol concentration so directly.

Metabolism. Your liver does almost all the work. Two enzymes do the heavy lifting:

  1. Alcohol dehydrogenase (ADH) breaks ethanol into acetaldehyde
  2. Acetaldehyde dehydrogenase (ALDH) breaks acetaldehyde into acetate

The acetate is then broken down further into water and carbon dioxide and excreted normally. The intermediate, acetaldehyde, is the toxic part.

Speed. Your liver processes alcohol at a roughly fixed rate of about one standard drink per hour. This rate doesn’t speed up much regardless of what you do. If you drink faster than your liver can process, alcohol builds up in your blood. If you drink slower, your blood alcohol stays manageable.

The rate varies between people due to genetics (some have more efficient enzymes), sex (women generally process slower), body composition, and liver health.

# Cause 1: Acetaldehyde toxicity

The intermediate metabolite acetaldehyde is far more toxic than ethanol. It’s classified as a probable human carcinogen by the WHO. In the short term, it causes:

  • Flushing (especially in faces)
  • Headache
  • Nausea and vomiting
  • Rapid heart rate
  • The “I feel poisoned” sensation

Acetaldehyde accumulates when ALDH (the second enzyme) can’t keep up with ADH (the first). This happens during heavy drinking sessions and is particularly pronounced in people with the ALDH2*2 genetic variant common in East Asian populations, who have a much slower-functioning ALDH enzyme. People with this variant get severe flushing, nausea, and elevated cancer risk from drinking, all driven by acetaldehyde accumulation.

For everyone else, acetaldehyde clears once the alcohol stops arriving. But during the night and into the early morning, while your liver is still processing the last few drinks, acetaldehyde levels remain elevated. This is much of why the morning feels physically toxic.

Laboratory glassware on a wooden surface, suggesting biochemistry.
Photo by Tara Winstead on Pexels

# Cause 2: Dehydration

Alcohol suppresses vasopressin (also called antidiuretic hormone, ADH, but a different ADH from the metabolic enzyme above). Vasopressin tells your kidneys to retain water. With less vasopressin, your kidneys flush water more aggressively.

The math:

  • A typical alcoholic drink contains 200-300ml of liquid
  • That same drink causes you to urinate roughly 320-500ml of water in the following hours

The net effect is a fluid deficit that compounds across a session. By the end of a heavy night, you can be 1-2 litres short on fluids. Your blood volume drops slightly, your kidneys are flushing electrolytes alongside the water, and your body’s sodium-potassium balance is disrupted.

The dehydration alone explains:

  • The dry mouth
  • The headache (cerebrospinal fluid pressure changes with dehydration)
  • The fatigue
  • The dizziness when you stand up
  • The general feeling of being depleted

This is why rehydration genuinely helps the morning after. Plain water replaces the fluid; electrolytes replace the salts you lost. We cover the practical side in Electrolytes and Hangovers.

# Cause 3: Glutamate rebound

This is the cognitive and emotional half of a hangover.

While alcohol is in your system, it does two things to your nervous system:

  • Enhances GABA, the inhibitory neurotransmitter that calms your brain
  • Suppresses glutamate, the excitatory neurotransmitter that activates your brain

Together, these produce the relaxation, the lowered inhibitions, the slowed thinking, the slurred speech.

When the alcohol leaves, your brain has to rebound. The GABA enhancement releases and the glutamate suppression releases simultaneously. But after hours of suppression, your brain has compensated by upregulating glutamate receptors. When glutamate floods back through hyper-sensitive receptors, you get a hyperactive nervous system. This is what produces:

  • Anxiety, dread, and panic (we cover this fully in Hangxiety Explained)
  • Racing thoughts
  • Sensitivity to noise and light
  • The wired-but-tired feeling
  • Some headache and tension component

The peak of glutamate rebound lands 12-18 hours after the last drink. This is why the morning often feels worse than the night.

# Cause 4: Sleep disruption

Alcohol is a sedative; it makes you fall asleep faster. But it disrupts sleep architecture once you’re asleep, particularly in the second half of the night. The mechanisms:

REM suppression. REM sleep (the dreaming phase, important for cognitive consolidation) is heavily reduced in the first half of the night after drinking. The brain compensates by attempting REM rebound in the second half, but the rebound is fragmented.

Increased awakenings. Alcohol metabolism produces small physiological alerting signals (raised heart rate, body temperature changes, hormone shifts) that fragment sleep. You may not fully wake up but you’re sleeping shallower.

Snoring and sleep apnoea. Alcohol relaxes the throat muscles that keep your airway open. People who don’t normally snore often snore after drinking; people who already have mild sleep apnoea get worse apnoea.

The total effect: you’ve spent time horizontal, but you haven’t actually rested properly. The fatigue you feel the next day is real sleep deprivation, not just hangover fatigue. We cover this in Hangovers and Sleep: Why You Wake Up at 4am.

# Cause 5: Inflammation

Alcohol triggers low-grade systemic inflammation. Your immune system responds to alcohol metabolism with elevated cytokines, particularly during the recovery phase. The inflammatory response:

  • Contributes to muscle aches and the general “unwell” feeling
  • Adds to brain fog (inflammation crosses into the brain)
  • Increases sensitivity to pain
  • Makes the recovery feel slower and worse than dehydration alone would explain

This is also why heavy drinkers experience accumulated inflammation effects over weeks and years, not just per-session. Each session adds to a baseline that’s already elevated.

Hands resting on a lap in soft natural light, suggesting calm and recovery.
Photo by cottonbro studio on Pexels

# Cause 6 (sometimes): Congeners

Congeners are byproducts of fermentation and ageing. Some are organic compounds that contribute to flavour and colour; some are metabolic byproducts of yeast. Darker drinks contain more congeners.

A rough ranking of congener content from low to high:

  • Pure vodka, white rum, gin (very low)
  • Light beer, white wine (low)
  • Dark beer, rosé (moderate)
  • Red wine, dark rum, bourbon, brandy (high)
  • Cheap whisky, port, fortified wines (very high)

Congeners contribute to hangover severity in several ways. Some are toxic in the same way acetaldehyde is, requiring liver processing. Some interfere with metabolism of the alcohol itself. Some have neurotoxic effects in their own right. The result: at the same volume of alcohol, dark drinks produce worse hangovers than clear drinks for most people.

We cover the chemistry in Why Wine Gives Worse Hangovers Than Vodka and the practical implications in Hangover-Free Drinks: Low-Congener Alcohol Choices.

# Why some people get worse hangovers than others

Putting it all together, hangover severity is a product of:

  • Volume of alcohol consumed (more alcohol = more of every cause above)
  • Pace of consumption (faster = higher peak BAC = more severe rebound)
  • Genetics of liver enzymes (slower ALDH = more acetaldehyde accumulation)
  • Hydration status going in (already dehydrated = worse the next day)
  • Sleep duration and quality (less rest amplifies everything)
  • Eating pattern (empty stomach = faster absorption = higher peak)
  • Drink choice (high-congener drinks add a layer of toxicity)
  • Age (liver enzyme efficiency declines, sleep architecture changes; we cover this in The 2-Day Hangover)
  • Sex (women metabolise alcohol slower per drink due to body composition and enzyme differences)
  • Underlying health (anxiety conditions, insulin resistance, inflammatory conditions all amplify hangover severity)

Two people drinking the same amount can have wildly different hangovers based on this combination. Understanding the variables helps you predict and adjust.

# Why most “hangover cures” don’t actually address the science

A useful sanity check: if a proposed cure doesn’t address one of the six mechanisms above, it probably doesn’t work. The few interventions that do address them:

  • Water and electrolytes address dehydration directly. Real benefit.
  • Sleep and rest address the sleep disruption. Real benefit.
  • Light food and slow caffeine support recovery without amplifying glutamate rebound. Real benefit.
  • NSAIDs (ibuprofen, aspirin) reduce inflammation and ease headache. Real but modest.
  • Time addresses everything by waiting it out. Most reliable.

What doesn’t work, and why:

  • Hair of the dog doesn’t accelerate clearance; it suspends the rebound by adding more alcohol, then resumes it later
  • Greasy food doesn’t neutralise acetaldehyde or glutamate; the perceived benefit is mostly placebo and the comfort of fullness
  • Hangover-cure pills sold in bars mostly don’t address any of the six mechanisms in any meaningful way, regardless of marketing claims
  • Pre-drink “hangover prevention” supplements have weak evidence; the few that have any effect (NAC, prickly pear) are minor adjuncts at best

We cover this in Hangover Cures That Don’t Work.

# How AlcoLog helps you understand your patterns

AlcoLog logs every drink with a timestamp, so you can see your pace across a session as it happens. The hydration reminder fires on a schedule (by time interval, by drink count, or both, set in Settings) so you remember to drink water alongside.

Over time, the History view’s calendar heatmap shows weekly drinking patterns, and the AlcoScore’s Recovery pillar factors in your rest days between sessions. Patterns that produce bad hangovers (long sessions, fast pace, high-congener drink choices, late nights) become visible in the data so you can adjust for next time.

Try AlcoLog free →

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