A hangover is a collection of unpleasant signs and symptoms that appear hours after heavy alcohol consumption, typically when the blood alcohol concentration begins to fall toward zero. These effects—which can include headache, nausea, fatigue, and general malaise—are caused by a complex interplay of processes within the body. This involves the body’s reaction to alcohol’s toxic byproducts, changes in fluid balance, the presence of impurities, and disruption to the sleep cycle. Understanding these biological mechanisms reveals why the day after drinking can feel so miserable.
Alcohol’s Toxic Byproduct
The primary explanation for many classic hangover symptoms lies in how the body processes ethanol. The liver first uses the enzyme alcohol dehydrogenase (ADH) to break down ethanol into acetaldehyde.
Acetaldehyde is a highly toxic chemical, and its buildup is strongly associated with symptoms like flushing, rapid heart rate, headache, and nausea. The liver normally relies on a second enzyme, aldehyde dehydrogenase (ALDH), to quickly convert acetaldehyde into the far less harmful substance, acetate.
When a person drinks heavily, the rate at which acetaldehyde is produced can outpace the rate at which ALDH can clear it, causing the toxic compound to accumulate. This accumulation directly irritates tissues and causes physical ailments. Genetic variations in ALDH enzyme efficiency can explain why some individuals experience intense reactions to alcohol.
Fluid Loss and Electrolyte Changes
Alcohol acts as a diuretic, causing the body to expel fluid at an accelerated rate, which leads to dehydration. This effect is triggered by alcohol suppressing the release of vasopressin (the antidiuretic hormone) from the pituitary gland. Without this hormone signaling the kidneys to reabsorb water, they send water directly to the bladder, resulting in increased urination.
This net fluid loss causes intense thirst and contributes to the throbbing headache often associated with a hangover. The headache occurs because the body attempts to compensate for systemic water loss by drawing fluid from the brain, which can cause the brain’s outer membranes to pull on the skull. Frequent urination also flushes out electrolytes like potassium and sodium, and their imbalance can cause fatigue and muscle cramps.
Impurities and the Immune System
Alcoholic beverages contain compounds other than ethanol, known as congeners, which are byproducts of the fermentation process. These substances, including chemicals like methanol and tannins, contribute significantly to the severity and duration of a hangover. Generally, darker liquors like brandy and red wine contain higher concentrations of congeners than lighter spirits like vodka, potentially leading to worse symptoms.
Alcohol also triggers an internal defense mechanism by activating the immune system. The body perceives alcohol and its metabolites as a threat, initiating an inflammatory response. This reaction involves releasing pro-inflammatory molecules called cytokines, which circulate in the bloodstream. Elevated cytokine levels are directly linked to the generalized “sick” feelings of a hangover, including malaise, muscle aches, and fatigue.
Disrupted Sleep and Brain Fog
While alcohol is a sedative that can help a person fall asleep quickly, it profoundly disrupts the quality and architecture of the sleep cycle. It suppresses rapid eye movement (REM) sleep, the stage of rest crucial for memory consolidation and emotional processing. This lack of restorative rest contributes to next-day cognitive symptoms like fatigue, irritability, and the inability to focus, commonly referred to as “brain fog.”
As the body metabolizes alcohol throughout the night, the initial sedative effect wears off, causing fragmented sleep and frequent awakenings. The nervous system experiences a rebound effect on neurotransmitters that contributes to anxiety and restlessness the following day. Specifically, the calming effects of GABA decrease, while the excitatory effects of glutamate increase, leading to the anxious, jittery feeling associated with a hangover.