EEG Test: Why It Is Done and What to Expect

01The test

What an EEG records

Mayo Clinic explains that brain cells communicate through electrical impulses and are active all the time, even during sleep. An EEG uses small metal discs called electrodes attached to the scalp to record this activity, which appears as wavy lines on the recording.

MedlinePlus explains that brain electrical activity has a number of waves per second (frequencies) that are normal for different levels of alertness: for example, brain waves are faster when awake and slower in certain stages of sleep. There are also normal patterns to these waves. MedlinePlus is clear that an EEG cannot be used to measure intelligence.

02Reasons

Why it is requested

The NHS explains that the main condition an EEG is used for is epilepsy, and a healthcare professional may refer you if you have symptoms such as seizures. Mayo Clinic and MedlinePlus list other situations where it may help:

• sleep conditions
• inflammation of the brain (encephalitis) and other brain diseases (encephalopathy)
• brain damage from a head injury, brain tumors or stroke
• fainting spells or periods of memory loss that cannot otherwise be explained, and behavioral changes
• coma and monitoring of the brain during surgery or anesthesia

An EEG is not a general screening test for every headache or faint: it is requested when the doctor needs information about brain electrical activity.

03Types

Types: routine, sleep-deprived, ambulatory, video

The NHS describes several types of EEG, whose duration depends on the type:

• routine EEG: a recording while you rest, usually lasting about an hour in total
• sleep EEG: done in hospital while you are asleep
• sleep-deprived EEG: done after you have been asked not to sleep for a certain time the night before
• ambulatory EEG: you wear a portable recorder during your usual activities; MedlinePlus mentions up to 3 days
• video telemetry EEG: video is recorded at the same time, usually over a few days in hospital

Mayo Clinic notes that an ambulatory EEG increases the chance of recording seizure activity, but is not as good as inpatient video EEG at distinguishing epileptic from nonepileptic seizures.

04Preparation

How to prepare: hair, medicines and sleep instructions

The three sources give similar advice:

• wash your hair the night before or the day of the test, and do not use conditioner, oils, sprays, gel or wax, as they make it harder for electrodes to stick
• remove weaves, braids or wigs if you wear them, or ask for specific instructions
• take your usual medicines unless your team tells you otherwise, and bring a list; do not stop or change any medicine without talking to your doctor
• MedlinePlus advises avoiding caffeine for 8 hours before the test
• for a sleep or sleep-deprived EEG, follow exactly the sleep instructions you are given, and avoid caffeine or energy drinks to stay awake

The NHS notes that you can usually eat and drink normally and that you can ask for a chaperone in the room. Do not deprive yourself of sleep or stop medicines unless the team has asked you to.

05Procedure

What happens during the test

Mayo Clinic and the NHS explain that a technician measures your head and marks the spots for the electrodes, which may be scrubbed lightly to improve the recording. Electrodes are attached with a sticky paste, or an elastic cap with electrodes is used, and connected by wires to the recording machine. You sit or lie comfortably, usually with your eyes closed, and need to stay still because movement can change the results.

Mayo Clinic notes that you feel little or no discomfort: the electrodes do not transmit any sensations, they only record. You may be asked to open and close your eyes, do simple calculations, read or look at a picture. Mayo Clinic gives 20 to 40 minutes of recording once electrodes are in place, and the NHS about an hour in total. Video is often recorded at the same time.

06Activation

Activation: breathing and flashing lights

During the recording, you may be asked to breathe deeply and quickly for a few minutes (hyperventilation) or to look at a flashing light.

MedlinePlus explains that the EEG is very safe, but flashing lights or fast breathing may trigger seizures in people with seizure disorders; Mayo Clinic notes that this is sometimes done intentionally, and the staff are trained to provide care if it happens. These maneuvers should only be done during the test, under supervision, not at home.

07After

After the test: driving and sedation

The electrodes or cap are removed and the scalp is cleaned; the NHS notes your hair may feel sticky until you wash the paste out, so a comb or brush is useful. If you did not have a sedative, you can usually return to your normal routine straight away.

If you had a sedative, Mayo Clinic advises arranging for someone to drive you home, resting and not driving for the rest of the day. The NHS adds that you will not be able to drive after a sleep-deprived EEG, so you should arrange to be picked up.

08Results

How results are read

Doctors trained to analyze EEGs interpret the recording and send a report to the professional who requested it. The NHS explains that results may take a few days or weeks and that you may need a follow-up appointment; if you have not heard after a few weeks, contact the referring doctor. Sometimes more tests, such as further EEGs, are needed.

MedlinePlus lists possible reasons for abnormal results, including seizure disorders such as epilepsy, bleeding or abnormal structures in the brain, stroke, head injury, brain infection, sleep disorders, migraines in some cases, medicines, alcohol or drug use and kidney or liver disease. An abnormal EEG is interpreted with your symptoms and history; it is not a diagnosis by itself. Mayo Clinic suggests asking what the next steps are, whether anything might have affected the result and whether the test needs repeating.

09Sources

Where the sources differ

MedlinePlus (January 2025) states clearly that a normal EEG does not mean that a seizure did not occur. This is one reason why some people need further EEGs, as the NHS notes, or longer ambulatory or video recordings, which Mayo Clinic explains increase the chance of recording seizure activity. Other sources describe the EEG as a way to “rule out” epilepsy, but a normal EEG alone is not enough to exclude it.

The sources also differ on details: Mayo Clinic (May 2024) gives 20 to 40 minutes of recording and the NHS (February 2026) about an hour in total; only MedlinePlus mentions avoiding caffeine. An EEG and an MRI answer different questions: the EEG records how the brain is working electrically during the test, while MRI shows the structure of the brain. Your doctor decides which is needed. MediBilbao Salud does not perform EEGs.

10Summary

In summary

An EEG records the brain’s electrical activity through electrodes on the scalp. It is painless and is used mainly in epilepsy, as well as in sleep conditions, brain inflammation and other brain problems. Come with clean hair without products, follow instructions on medicines, caffeine and sleep, and do not drive after sedation or a sleep-deprived EEG.

Results are read by a specialist and interpreted with your symptoms. A normal EEG does not rule out a past seizure, so describing to your doctor exactly what happened remains important. Do not stop medicines or try sleep deprivation or breathing maneuvers at home unless you have been told to.