An EEG, or electroencephalogram, is a test that records the electrical activity of your brain using small sensors placed on your scalp. It’s one of the most common neurological tests, primarily used to diagnose epilepsy and other seizure disorders, but it also helps evaluate a wide range of brain conditions from sleep disorders to encephalopathy. The test is painless, noninvasive, and typically takes 45 minutes to 2 hours.
How an EEG Records Brain Activity
Your brain contains billions of neurons that communicate through tiny electrical signals. An EEG picks up the combined electrical activity of large groups of neurons firing in sync, specifically from cells in the outer layer of the brain (the cortex) that are oriented perpendicular to the surface of the skull. The electrodes don’t penetrate the skin. They sit on the scalp and detect the sum of all the excitatory and inhibitory signals happening beneath them.
The result is a continuous readout of wavy lines, each representing electrical activity from a different region of the brain. A neurologist reads these patterns to determine whether your brain activity looks normal or shows signs of a specific condition.
Brain Wave Patterns and What They Mean
Brain waves are categorized by their speed, measured in cycles per second (Hertz). Each type corresponds to a different state of consciousness.
- Delta waves (0.5 to 4 Hz) are the slowest and appear during deep sleep. They’re most prominent over the front and center of the head.
- Theta waves (4 to 7 Hz) show up during drowsiness and light sleep stages.
- Alpha waves (8 to 12 Hz) are characteristic of relaxed wakefulness. They’re strongest at the back of the head when your eyes are closed and typically fade when you open your eyes or concentrate.
- Beta waves (13 to 30 Hz) are the fastest common rhythm and dominate during normal waking activity in adults and children. Their intensity increases slightly during drowsiness, then drops off as sleep deepens.
When an EEG detects abnormal patterns, such as spikes, sharp waves, or spike-and-slow-wave complexes, these can indicate epileptic activity. Spikes are brief bursts of electrical activity lasting 20 to 70 milliseconds, while sharp waves last slightly longer. These patterns help pinpoint not only whether someone has epilepsy but also where in the brain seizures originate and what type of epileptic syndrome they may have.
What an EEG Is Used For
Epilepsy evaluation is the most well-known use for EEG. The test helps classify seizure types, locate where seizures start in the brain, and monitor ongoing seizure activity. It’s also essential for managing status epilepticus, a dangerous condition where seizures don’t stop on their own.
Beyond epilepsy, EEGs serve a surprisingly wide range of purposes. Doctors use them to evaluate unexplained changes in consciousness or mental status, assess the severity of brain dysfunction caused by metabolic problems or toxic exposures, and investigate episodes of fainting when heart-related causes have been ruled out. In intensive care, continuous EEG monitoring tracks brain function in comatose or unresponsive patients and helps guide the depth of medically induced comas. EEGs also play a role in diagnosing brain tumors, brain inflammation (encephalitis), stroke, sleep disorders, dementia, and neurodegenerative conditions like Creutzfeldt-Jakob disease and Huntington’s disease. In end-of-life situations, EEG is one of the tools used to confirm brain death.
One thing worth knowing: a routine EEG has limited sensitivity for epilepsy. Only 10 to 27% of adults show epilepsy-related abnormalities on their first EEG after a seizure. Getting the test done early after a seizure event improves those detection rates, and repeat or longer recordings can catch what a single routine test misses.
Types of EEG Tests
Not every EEG looks the same. The type your doctor orders depends on what they’re trying to find.
A routine EEG is the standard version, lasting roughly 45 minutes to 2 hours in a clinic or hospital. You sit or lie still while the technician records your brain activity. During the test, you may be asked to breathe deeply for a few minutes or look at a flashing light. These “activation” techniques can bring out abnormal brain patterns that wouldn’t otherwise appear.
A sleep EEG records your brain activity while you sleep, usually over several hours. Some brain abnormalities only emerge during sleep, so this version captures what a waking test cannot. Sleep EEGs are sometimes combined with a full sleep study to evaluate sleep disorders.
An ambulatory EEG lets you leave the hospital. You wear a small, portable recording device connected to electrodes on your scalp for one to three days, going about your normal routine. If you experience a seizure or unusual event, you or someone with you can press a button on the device to mark it. This type gives doctors a much fuller picture of your brain activity during everyday life and can spare you a hospital stay.
A video EEG is done in an inpatient epilepsy monitoring unit, where cameras record you continuously alongside the EEG data. This allows doctors to match visible seizure behavior with the electrical patterns happening in your brain at the same moment, which is critical for confirming a diagnosis, planning surgery, or distinguishing epileptic seizures from episodes that look similar but have a different cause.
How to Prepare
Preparation is straightforward. Wash your hair with shampoo the night before, but skip conditioner, hairspray, gel, or any styling products. These can interfere with the electrodes’ ability to pick up signals. Avoid caffeine for 8 to 12 hours before the test, since it alters brain wave patterns. Your doctor may also ask you to sleep less than usual the night before if a sleep-deprived recording would be helpful.
What the Test Feels Like
An EEG technician places about 20 small, flat electrodes on your scalp using a washable paste or adhesive. The setup takes a bit of time, but the electrodes don’t hurt. Once recording starts, you’ll be asked to relax, keep still, and follow simple instructions like opening and closing your eyes. You may be asked to breathe deeply for a few minutes or watch a strobe light, both of which are standard parts of the exam designed to provoke certain brain responses.
The test carries essentially no physical risk. The electrodes only detect electrical activity; they don’t deliver any current to your brain. In rare cases, the deep breathing or flashing light can trigger a seizure in someone who is prone to them, but this happens in a controlled clinical setting with trained staff present. After the test, you can wash the paste out of your hair and go about your day normally.