Seizures happen when nerve cells in the brain fire electrical signals in an uncontrolled, synchronized burst. The lifetime risk of experiencing at least one seizure is 8% to 10%, making this far more common than most people realize. The causes range from temporary triggers like fever or low blood sugar to chronic conditions like epilepsy or brain injury, and sometimes no cause is ever identified.
What Happens in the Brain During a Seizure
Your brain runs on a careful balance between signals that excite nerve cells and signals that calm them down. The main excitatory chemical is glutamate, which tells neurons to fire. The main inhibitory chemical is GABA, which tells them to stop. A seizure occurs when this balance tips too far toward excitation, either because glutamate levels surge or GABA levels drop. The result is a wave of excessive electrical activity that can stay in one part of the brain (a focal seizure) or spread across both hemispheres (a generalized seizure).
This imbalance can be triggered by dozens of different underlying problems. Some are structural, like scar tissue or a tumor pressing on brain cells. Others are chemical, like a sudden drop in blood sugar. And some are genetic, involving inherited differences in how ion channels on nerve cells open and close. What they all share is the same endpoint: too many neurons firing at once.
Metabolic and Electrolyte Imbalances
Your brain is extremely sensitive to the chemical composition of your blood. When levels of glucose, sodium, calcium, or magnesium swing outside their normal range, the electrical stability of nerve cells breaks down.
Low blood sugar (hypoglycemia) is one of the most common metabolic seizure triggers, particularly in people with diabetes who use insulin. The brain depends almost entirely on glucose for fuel, and when supply drops sharply, neurons malfunction.
Sodium imbalances are another major cause. Seizures from low sodium (hyponatremia) typically occur when levels plummet rapidly below 115 milliequivalents per liter, though the speed of the drop matters as much as the number itself. Rapid swelling of brain cells is what actually triggers the seizure. High sodium above 158 to 160 can cause seizures too, especially when the rise happens within hours. Low magnesium, usually at levels below 1 milliequivalent per liter, and calcium abnormalities in either direction can also provoke generalized tonic-clonic seizures, the type involving full-body stiffening and shaking.
These metabolic seizures are considered “provoked,” meaning they have an identifiable, reversible trigger. Correcting the underlying imbalance usually stops them from recurring.
Fever in Young Children
Febrile seizures affect 2% to 5% of children in the U.S. and Europe, typically between the ages of 6 months and 5 years, with peak risk around 12 to 18 months. They occur alongside a fever above 100.4°F (38°C), though there is no single temperature threshold that applies to every child. Some children seize at 101°F while others tolerate much higher fevers without any neurological symptoms.
The seizure itself is usually brief and generalized, and it does not mean a child has epilepsy. The developing brain is simply more vulnerable to the rapid temperature changes that accompany common childhood infections. Most children outgrow the susceptibility entirely by age five.
Alcohol and Drug Withdrawal
Alcohol withdrawal is one of the most dangerous and well-recognized seizure triggers. After prolonged heavy drinking, the brain adapts to the constant presence of alcohol by ramping up excitatory signaling. When alcohol is suddenly removed, that excess excitation goes unchecked.
Withdrawal seizures typically appear 6 to 48 hours after the last drink, and more than 90% occur within the first 48 hours. They are usually generalized tonic-clonic and can cluster in groups. This is a medical emergency because it can progress to a life-threatening condition called delirium tremens. Withdrawal from benzodiazepines and certain other sedatives carries a similar risk through the same basic mechanism.
Traumatic Brain Injury
A blow to the head can cause seizures immediately, within days, or years later. The severity of the injury is the strongest predictor. A large Norwegian study found that within five years of a traumatic brain injury, 13.2% of people with severe injuries developed epilepsy, compared to about 4% of all TBI patients combined. Even mild TBI raised the risk roughly fourfold compared to people without any head trauma.
Early seizures, occurring within the first week, are thought to result from direct physical disruption of brain tissue, bleeding, or swelling. Late seizures, emerging weeks to years later, are driven by scar tissue formation and long-term changes in how neural networks are wired. Having an early seizure after a head injury significantly increases the odds of developing epilepsy down the road.
Stroke and Vascular Problems
About 7% of stroke patients experience a seizure within the first week, and roughly 10% go on to develop post-stroke epilepsy over time. The risk is highest with hemorrhagic strokes (bleeding in the brain), severe strokes, and strokes that involve the outer surface of the brain (the cortex). Cortical involvement triples the odds compared to deeper strokes.
Early post-stroke seizures are driven by the acute injury itself: swelling, chemical release from dying cells, and disrupted blood flow. Late seizures, appearing more than a week after the stroke, result from the scar tissue that forms as the brain heals. That scar tissue creates pockets of abnormal electrical activity that can fire unpredictably. Having a seizure in the first week after a stroke increases the risk of later epilepsy by about sevenfold.
Brain Tumors
Seizures are often the first noticeable symptom of a brain tumor. The incidence varies enormously by tumor type, ranging from 10% to over 80%. Slow-growing tumors in the brain’s outer layers, particularly certain types of gliomas and glioneuronal tumors, carry the highest seizure rates. These tumors disrupt normal signaling by compressing surrounding tissue, altering local blood supply, and changing the chemical environment around neurons.
Brain Infections
Infections that reach the brain or its surrounding membranes are potent seizure triggers. Viral encephalitis (inflammation of the brain itself) carries the greater long-term risk: people who have early seizures during the infection face a 22% chance of developing unprovoked seizures over the next 20 years. Even without early seizures, the 20-year risk is 10%.
Bacterial meningitis is somewhat less likely to cause lasting seizure disorders, with a 20-year risk of about 2.4% for those who didn’t seize during the acute illness, rising to 13% for those who did. The infection damages brain tissue through inflammation, swelling, and in some cases direct destruction of neurons, leaving behind the same kind of scarring that drives seizures after stroke or trauma.
Genetic Causes
Some people are born with gene mutations that make their neurons inherently more excitable. One of the best-studied examples involves the SCN1A gene, which provides instructions for building sodium channels on nerve cells. Mutations in this gene cause a wide spectrum of seizure disorders. At the mild end, a person might experience simple febrile seizures that resolve in childhood. At the severe end, the same gene causes Dravet syndrome, a form of epilepsy that begins in infancy and is extremely difficult to control with medication.
SCN1A is just one of many epilepsy-related genes. Others affect potassium channels, calcium channels, or the receptors that respond to GABA and glutamate. Genetic epilepsies often show up in the first months or years of life, though some don’t become apparent until adolescence. A family history of seizures raises suspicion, but many genetic cases arise from new mutations not present in either parent.
Sleep Deprivation and Other Lifestyle Triggers
For people who already have a lower seizure threshold, whether from epilepsy, a previous brain injury, or genetic predisposition, certain everyday factors can push the brain past its tipping point. Sleep deprivation is one of the most reliable triggers. Going without sleep increases cortical excitability in a measurable way, which is why sleep-deprived EEGs are used as a diagnostic tool to provoke abnormal brain activity in a controlled setting.
Other common triggers include extreme stress, flashing or flickering lights (in people with photosensitive epilepsy), missed meals, dehydration, and stimulant drugs like cocaine and amphetamines. These don’t cause seizures in most healthy people, but they can be the final nudge for a brain that’s already vulnerable.
When No Cause Is Found
In a significant number of cases, particularly first-time seizures in otherwise healthy adults, no clear cause is identified despite thorough testing. These are classified as unprovoked seizures. A single unprovoked seizure doesn’t necessarily mean a person has epilepsy. The diagnosis of epilepsy requires either two or more unprovoked seizures separated by at least 24 hours, or one seizure combined with evidence (from brain imaging or electrical recordings) that the risk of recurrence is high. About half of people who have a single unprovoked seizure never have another one.