Electroencephalography, or EEG, is a test that records the electrical activity of your brain through small sensors placed on your scalp. It’s the primary tool doctors use to diagnose epilepsy and other seizure disorders, and it plays a key role in evaluating sleep conditions, brain inflammation, and head injuries. The test is painless, noninvasive, and typically takes about 30 minutes for a routine recording.
How Your Brain Produces Electrical Signals
Your brain’s billions of nerve cells communicate by passing electrical and chemical signals to one another. When one neuron sends a signal to the next, it creates a tiny flow of current at the connection point between them. These currents, generated by synaptic activity, are the primary source of what an EEG picks up. Individual action potentials (the sharp electrical “spikes” that travel along a single nerve fiber) contribute negligibly to what reaches the scalp.
No single neuron produces enough electricity to detect from outside the skull. What the EEG actually measures is the combined electrical field created by large populations of neurons firing in sync. Each neuron acts like a tiny battery with a positive and negative end, forming what physicists call a dipole. When thousands or millions of these dipoles line up and fire together, their signals add up and travel through brain tissue, skull, and skin to the surface, where electrodes can detect them.
What the Different Brain Waves Mean
EEG signals are categorized by their frequency, measured in cycles per second (hertz). Each frequency band corresponds loosely to a different brain state.
- Delta (0.5–4 Hz): The slowest waves, prominent during deep sleep. In an awake adult, significant delta activity can signal a problem.
- Theta (4–7 Hz): Appears during drowsiness and light sleep. In children and young adults, bursts of theta can also show up during heightened emotional states.
- Alpha (8–12 Hz): The signature rhythm of a relaxed, awake brain with eyes closed. Opening your eyes or concentrating on a task suppresses alpha waves almost immediately.
- Sigma (12–16 Hz): Known as sleep spindles, these brief bursts define stage 2 sleep and are thought to play a role in memory consolidation.
- Beta (13–30 Hz): The most common rhythm in alert, awake adults and children. Beta activity increases during active thinking and problem-solving.
- Gamma (30–80 Hz): The fastest waves, linked to sensory processing and higher-level cognition.
During a routine EEG, the technologist looks at which frequencies dominate, where on the scalp they appear, and whether any abnormal patterns (like sharp spikes or unusually slow waves) show up. The alpha rhythm, for instance, should be strongest over the back of the head. If it’s absent or asymmetric, that’s a clue worth investigating.
What an EEG Is Used to Diagnose
Epilepsy is the most common reason doctors order an EEG. The test can detect characteristic spike-and-wave patterns that indicate a seizure disorder, even between seizures. Beyond epilepsy, EEGs help evaluate brain damage from head injuries, encephalopathy (a broad category of brain dysfunction from metabolic, toxic, or infectious causes), brain tumors, stroke, and inflammatory conditions like herpes encephalitis. The test is also used to confirm brain death in patients in a coma and to monitor anesthesia depth during a medically induced coma.
A routine 30-minute EEG has high specificity for epilepsy in adults (about 95%), meaning a positive result is very reliable. But its sensitivity is low: only around 17% of adults with epilepsy will show abnormal activity during a single short recording. Children fare somewhat better, with sensitivity around 58%. The reason is simple. If you aren’t having abnormal electrical activity during the brief window of the test, the EEG will look normal even if you have epilepsy. That’s why doctors sometimes order repeat tests or longer recordings.
Ambulatory and Extended Monitoring
When a standard EEG doesn’t capture enough information, a 24-hour ambulatory EEG can be far more revealing. You wear a portable recording device home and go about your day (and night) while the EEG runs continuously. In one study of adults after a first unprovoked seizure, the 24-hour ambulatory recording detected abnormal activity with 72% sensitivity, compared to just 11% for a single routine EEG and 22% for a second routine EEG. The extended recording time dramatically increases the odds of catching an abnormal event.
What Happens During the Test
Electrodes are placed on your scalp using a standardized map called the International 10-20 system. The name comes from the spacing: electrodes sit at intervals of 10% or 20% of the total distance between specific skull landmarks. This ensures consistent placement across patients and hospitals. A technologist measures your head, marks the positions, and attaches each small metal disc with a conductive paste or gel that helps the signal travel from skin to sensor. Most setups use 21 electrodes, though some use more for specialized recordings.
The whole preparation takes about 20 to 30 minutes. Once the electrodes are in place, you’ll typically lie back or sit in a comfortable chair. The recording itself is painless. You’ll be asked to stay still, close your eyes, and relax for stretches of time. The technologist may ask you to breathe deeply and rapidly (hyperventilation) for a few minutes, which can bring out certain abnormal patterns. You may also be exposed to a flashing strobe light at different speeds, a technique called photic stimulation that tests for light-sensitive seizure activity.
Photic stimulation provokes an abnormal photoparoxysmal response in about 1.5% of patients. Actual seizures occur in roughly 0.7% of cases, and generalized tonic-clonic seizures (the most severe type) happen in only 0.04%, making the technique very safe overall. If a seizure does begin, the technologist stops the stimulation immediately.
How to Prepare
Preparation is straightforward. Wash your hair thoroughly before the appointment and skip conditioners, hairsprays, gels, and oils, because residue on the scalp interferes with electrode contact. If you’re scheduled for a sleep-deprived EEG (designed to record your brain transitioning into sleep), you’ll be asked to sleep only until midnight the night before and stay awake from then until your appointment. Avoid caffeine after midnight as well, since it can prevent you from falling asleep during the test. For a standard sleep EEG, simply skipping caffeine on the day of the test is usually enough.
Continue taking your regular medications unless your doctor specifically tells you otherwise. Stopping seizure medications without guidance can be dangerous.
EEG and Sleep Studies
EEG is the backbone of sleep studies (polysomnography). Each sleep stage has a distinct electrical signature. As you drift from wakefulness into stage 1 sleep, the alpha rhythm drops out and is replaced by slower activity. Slow rolling eye movements, typically at 0.25 to 0.5 Hz, are often the first visible sign of drowsiness. Sharp, high-voltage waves called vertex waves appear at the top of the head, with amplitudes of 50 to 150 microvolts.
Stage 2 sleep, the stage where you spend most of the night, is defined by the appearance of sleep spindles: brief bursts of rhythmic activity in the 12 to 16 Hz range. K complexes, large sharp waveforms, also appear and are often followed by a spindle. In stages 3 and 4 (deep sleep), slow delta waves dominate. REM sleep, where most dreaming occurs, produces a pattern that looks surprisingly similar to wakefulness on EEG, which is why it’s sometimes called “paradoxical sleep.” REM typically doesn’t appear during a routine 30-minute EEG because the first REM period normally begins about 100 minutes after falling asleep.
A Brief Origin Story
The German psychiatrist Hans Berger made the first EEG recording from a human brain on July 6, 1924, during a neurosurgical operation on a 17-year-old boy. He spent five years refining and validating his results before publishing them in 1929, introducing the terms “alpha waves” and “beta waves” that are still used today. At the time, there were no CT scans, no MRIs, no modern imaging tools. EEG was one of the only windows into a living brain’s activity, and it transformed both neurology and psychiatry.