Seizures happen when nerve cells in the brain fire electrical signals in an abnormal, synchronized burst. Up to 10% of people worldwide will have at least one seizure during their lifetime, and the causes range from genetic conditions and brain injuries to chemical imbalances and high fevers. Some people develop recurring seizures (epilepsy), while others experience a single episode triggered by a temporary, reversible cause.
What Happens in the Brain During a Seizure
Your brain cells communicate through carefully timed electrical impulses. Each nerve cell fires, resets, and waits for the next signal. During a seizure, groups of neurons become hyperexcitable: a single stimulation triggers a rapid-fire chain of electrical discharges with no rest period between them. Instead of one controlled signal, you get a storm of activity that can spread across the brain.
This hyperexcitability often comes down to a problem with ion channels, tiny gateways on the surface of nerve cells that control the flow of charged particles in and out. When these channels malfunction, the cell membrane becomes too permeable and can’t return to its resting state. The brain also relies on a balance between signals that excite neurons and signals that calm them down. When the excitatory side overwhelms the calming side, seizure activity can begin.
Genetic and Inherited Causes
Some people are born with gene mutations that directly affect how their brain cells function. One well-studied example involves mutations in the SCN1A gene, which plays a key role in how neurons manage electrical signals. About 90% of children with Dravet syndrome, a severe form of epilepsy, carry a mutation in this gene. Seizures in Dravet syndrome typically begin between 4 and 12 months of age.
SCN1A is just one of many genes linked to seizure disorders. Some mutations cause epilepsy on their own, while others simply lower a person’s seizure threshold, making them more vulnerable to triggers that wouldn’t affect someone else. Family history of epilepsy raises risk, though many people with a genetic predisposition never have a seizure unless another factor, like illness or sleep deprivation, tips the balance.
Brain Injuries and Structural Damage
Any physical damage to brain tissue can create areas of abnormal electrical activity. Traumatic brain injuries from falls, car accidents, or sports collisions are a significant cause. Early post-traumatic seizures, those occurring within the first week after a head injury, happen in roughly 0.4 to 10% of cases overall, but that figure climbs to 20 to 25% in people with severe injuries. These early seizures are considered a direct reaction to the injury rather than a sign of epilepsy, though some people go on to develop recurring seizures months or years later.
Stroke is another common structural cause. In one large community study following 469 people after an initial stroke caused by a blood clot, 5.3% developed seizures. The damaged brain tissue left behind by a stroke can become a permanent source of abnormal electrical signals. Brain tumors, whether cancerous or benign, can also trigger seizures by pressing on surrounding tissue or disrupting normal neural circuits.
Infections That Affect the Brain
When an infection causes inflammation in the brain itself, seizures are a common result. Encephalitis, or swelling of the brain, can be caused by a range of viruses including herpes simplex virus (the most common cause of severe cases), the varicella-zoster virus behind chickenpox and shingles, and mosquito-borne viruses like West Nile. Tick-borne viruses, enteroviruses, and in rare cases bacteria, fungi, or parasites can also be responsible.
Meningitis, an infection of the membranes surrounding the brain and spinal cord, carries similar risks. In these cases, the seizures are driven by the inflammatory response itself: swelling, pressure changes, and direct irritation of brain tissue all contribute to abnormal electrical firing. Treating the underlying infection typically resolves the seizures, though some people sustain lasting brain damage that leads to epilepsy.
Metabolic and Chemical Imbalances
Your brain is extremely sensitive to changes in blood chemistry. When levels of sodium, glucose, calcium, or other essential substances shift far enough from normal, the electrical environment in the brain becomes unstable.
Low blood sodium (hyponatremia) is one of the most common metabolic triggers. Seizures typically occur when sodium drops below 115 milliequivalents per liter, well below the normal range of around 136 to 145. If the drop happens rapidly, seizures can occur at higher levels. On the other end, sodium above 160 can also cause neurological problems, and levels above 180 are often fatal.
Low blood sugar is another well-recognized trigger. Seizures from hypoglycemia generally occur when glucose falls below 40 mg/dL, a level far below the normal fasting range of 70 to 100. This is most commonly seen in people with diabetes who take too much insulin or miss a meal, but it can happen in anyone whose blood sugar drops severely. Extremely high blood sugar, above roughly 600 mg/dL, can also provoke seizures through a different mechanism involving dehydration of brain cells.
Alcohol Withdrawal
Heavy, long-term alcohol use changes the brain’s chemistry. Alcohol enhances the calming signals in the brain while suppressing the excitatory ones. When a heavy drinker suddenly stops, the brain is left in a state of extreme overexcitement with its calming systems suppressed. The result can be generalized tonic-clonic seizures, the type involving full-body convulsions and loss of consciousness.
These seizures most commonly occur 24 to 48 hours after the last drink, though symptoms of withdrawal have been reported as early as two hours after cessation. Alcohol withdrawal seizures are a medical emergency and one of the more dangerous causes of seizures in adults. Notably, alcohol withdrawal can also trigger seizures in people who have had a head injury, regardless of how severe the injury was.
Febrile Seizures in Children
Febrile seizures are the most common type of seizure in young children, occurring between the ages of 6 months and 5 years. They’re triggered by fever of 100.4°F (38°C) or higher, without any underlying brain infection. Despite a persistent belief that rapidly spiking fevers are the culprit, the risk is actually tied to how high the temperature gets, not how quickly it rises. Each child has a different seizure threshold, which is why some children with high fevers never have a seizure while others do.
Febrile seizures are frightening to witness but are generally not harmful. Most last less than a few minutes and do not cause brain damage or increase the risk of epilepsy later in life, though a small percentage of children who have complex or prolonged febrile seizures may face slightly elevated risk.
Sleep Deprivation and Lifestyle Triggers
For people who already have a lower seizure threshold, whether from genetics, a previous brain injury, or an existing seizure disorder, certain everyday factors can push them over the edge. Sleep deprivation is one of the most reliable triggers. Missing even one night of sleep can provoke a seizure in someone with epilepsy, and it’s also a recognized trigger for first-time seizures in people who didn’t know they were vulnerable.
Extreme stress, stimulant drug use, and certain medications that lower the seizure threshold (some antidepressants and antipsychotics, for example) can also contribute. Photosensitive seizures affect a smaller group of people, typically those with a specific type of epilepsy. Flashing lights between 5 and 30 flashes per second are most likely to trigger these seizures, which is why video games, strobe lights, and certain video content carry warnings.
When No Cause Is Found
In a significant number of epilepsy cases, no clear structural, metabolic, or infectious cause can be identified. These cases are sometimes called idiopathic epilepsy, and they’re often rooted in subtle genetic variations affecting ion channels in brain cells. The membrane surrounding the neuron becomes too permeable, preventing it from resetting properly after firing. The result is the same pattern of hyperexcitability seen in other forms of epilepsy, just without an obvious external trigger.
Having a single seizure doesn’t mean you have epilepsy. Epilepsy is defined as a tendency toward recurring, unprovoked seizures. Many people have one seizure from a temporary cause, like a metabolic imbalance, a high fever, or alcohol withdrawal, and never have another once that cause is addressed.