A hangover is caused by the interaction of alcohol metabolism, inflammatory responses, neurotransmitter imbalances, and fluid loss. Here you’ll find the biochemical processes and typical time windows explained in an understandable way.
What Happens in the Body During a Hangover? #
A hangover is not caused by a single trigger. It results from a complex mix of toxins such as acetaldehyde, disruption of brain neurotransmitters, inflammatory processes, sleep disturbances, and loss of water and electrolytes. These mechanisms occur at different times, which explains why symptoms are often only fully felt the next morning.
1. Alcohol metabolism and acetaldehyde #
Onset: during drinking until a few hours afterwards
During alcohol breakdown, acetaldehyde is produced first – a highly reactive intermediate and a substance widely used in the chemical industry. It irritates mucous membranes, increases heart rate, and activates inflammatory pathways. Even though the body quickly converts acetaldehyde further into acetate, brief concentration peaks are enough to set the stage for headache, nausea, and inner restlessness.
2. Acetate, energy metabolism, and lactate increase #
Time window: 2 to 10 hours after last drink
The acetate formed from acetaldehyde alters energy metabolism. The body temporarily switches to a less efficient form of energy production, leading to:
• relative hypoglycaemia
• increased lactate production
• disturbances in acid-base balance
These effects intensify fatigue, weakness, and headaches.
3. Immune system inflammatory response #
Peak: 12 to 24 hours after last drink
Alcohol activates the immune system, including in the brain. Microglia release increased amounts of pro-inflammatory messengers (e.g. IL-6, TNF-α). The result feels like a mild infection: head and body aches, exhaustion, difficulty concentrating, and heightened pain sensitivity.
4. Neurotransmitter rebound during the night #
Onset: 3 to 6 hours after last drink
While intoxicated, alcohol enhances GABA (inhibitory) and suppresses glutamate (excitatory). When blood alcohol levels drop, the opposite occurs:
• the calming GABA effect collapses
• glutamate rises above normal levels
The body slips into a state of hyperexcitability. Typical consequences: restless sleep, palpitations, irritability, noise sensitivity, and increased stress the next morning.
5. Dehydration and electrolyte imbalance #
Onset: 1 to 3 hours after last drink
Alcohol inhibits antidiuretic hormone (ADH). The body loses more water and minerals. Sodium and potassium in particular become imbalanced. Common symptoms include thirst, dizziness, dry mucous membranes, and throbbing headaches.
6. Residual alcohol remaining in the blood #
Duration: depending on amount, 10 to 20 hours
Even if you feel “sober,” measurable residual alcohol may still be present. As long as ethanol is circulating, many of the processes described above continue. Only when blood alcohol drops close to zero do the counter-reactions and inflammatory processes fully unfold. That’s why the hangover often feels worst when the alcohol is almost completely metabolised.
Typical overall course of a hangover #
• Onset of symptoms: 6 to 12 hours after last drink
• Peak: 12 to 24 hours
• Resolution: 24 to 36 hours, in some cases up to 48 hours
While alcohol is still in the blood, it has a dampening effect and masks many symptoms. Only when the level drops and the over-compensating neurotransmitters kick in do headache, restlessness, and fatigue fully hit.
Frequently Asked Questions About Hangovers (FAQ) #
Why does a hangover feel like having the flu?
Alcohol triggers a measurable inflammatory response. Cytokines such as IL-6 and TNF-α rise and produce symptoms similar to a mild infection—fatigue, head and body aches, etc.
The rebound of excitatory neurotransmitters like glutamate causes very restless, superficial sleep despite long sleeping time. The body recovers poorly, making the hangover worse.Why do I wake up so unrefreshed after drinking?
Drinking enough water reduces dehydration but does not prevent the neurotransmitter rebound or inflammatory response. The hangover can be milder, but not reliably prevented.Can you prevent a hangover by drinking plenty of water?
With age, enzyme activity, body water content, and sleep quality decline. This makes alcohol and its metabolites act more intensely, resulting in stronger hangovers.Why are hangovers often worse with age?
Bernd Guzek, MD, PhD #
Physician, author & co-founder of Bye-Bye-Booze
Specialized in biochemical mechanisms of addiction and brain metabolism.
