Electroencephalogram (EEG): Types, How to Prepare and What Results Mean

Written and medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: November 2026 · How we review

Quick Answer: What is an EEG?

An electroencephalogram (EEG) is a painless test that records the brain's electrical activity through electrodes placed on the scalp. It helps providers diagnose epilepsy and look into symptoms such as confusion, fainting or memory loss. A routine EEG takes 20 to 30 minutes, while other types record for an hour, a full day or longer. Side effects are rare. [1]

What the research says

• An abnormal EEG helps predict another seizure. An evidence-based guideline found that after a first unprovoked seizure in adults, the risk of another seizure is highest in the first two years (21% to 45%), and an EEG showing epileptiform abnormalities is one of the factors linked to a higher risk. (Krumholz et al., Neurology, 2015) [2]

• Longer recordings catch more. In 95 people who had a routine EEG and then a multi-day ambulatory EEG (median 3.9 days), abnormal findings rose from 18% to 33%, and seizures were captured in 8% of ambulatory recordings versus 3% of routine ones. (Nurse et al., Epilepsy & Behavior, 2024) [3]

Key takeaways

• An EEG records brain activity as wave patterns using about 23 scalp electrodes. [1]

• It is the main test for seizures and suspected epilepsy. [1]

• There are five main types: routine, prolonged, ambulatory, video and sleep EEG. [1]

• Wash your hair the night before and skip conditioner and styling products. [1]

• Side effects are rare and usually minor. [1]

What is an EEG?

An EEG records the electrical activity of your brain. It is painless and can be done while you are awake or asleep. [1]

Electrodes on your scalp pick up signals from your brain cells and send them to a machine that shows them as brain waves. A provider then reviews the recording for abnormal patterns. [1]

Why would you need an EEG?

Providers may order an EEG if you have had seizures or might have epilepsy. They may also order one for symptoms such as: [1]

• Confusion

• Fainting

• Memory loss

• Changes in behavior

An EEG can also monitor brain activity in conditions such as dementia, encephalitis, brain tumors, coma and traumatic brain injury. [1]

What are the types of EEG?

The type you need depends on what your provider is looking for.

Type	How it works	How long
Routine EEG	May use flashing lights or deep breathing to bring out patterns	About 20 to 30 minutes
Prolonged EEG	A longer office or hospital recording	About an hour or longer
Ambulatory EEG	A small portable recorder while you go about your day	One or more days
Video EEG	Records video and audio alongside brain activity	Varies
Sleep EEG	Often done with a sleep study when a sleep disorder linked to epilepsy is suspected	Varies

Sources: [1]

A video EEG helps your provider see what happens in your body during a seizure. [1]

How do you prepare for an EEG?

Your provider will give you instructions. In general: [1]

• Tell your provider about all the medicines you take.

• Wash your hair the night before. Do not use conditioner or styling products.

• Follow any instructions about sleep. You may be asked to stay awake the night before.

• Follow any instructions about eating or drinking.

• Ask whether you need a ride home, since some types of EEG require one.

What happens during a routine EEG?

A technician attaches about 23 electrodes to your scalp with glue or paste. The electrodes do not hurt. [1]

You rest with your eyes open or closed. The technician may ask you to look at a flashing light or breathe deeply for a few minutes. A routine EEG typically takes 20 to 30 minutes. [1]

Are there side effects?

Side effects are rare. Possible effects include: [1]

• Dizziness while breathing deeply

• Minor, short-lived hair loss or skin irritation where electrodes were placed

• In people with certain types of epilepsy, a seizure triggered by deep breathing or flashing lights, which is uncommon

What happens after an EEG?

The technician removes the electrodes, and you can usually go back to your normal activities. Your hair may feel sticky, so you may want to wash it. [1]

What do EEG results mean?

A provider reviews your recording for abnormal patterns. You will have a follow-up appointment to go over the results. If they are abnormal, you may be referred to a specialist such as a neurologist. [1]

EEG vs. functional MRI

An EEG shows brain activity as waves over time, while a functional MRI shows similar activity as images of the brain. [1]

Conclusion

An EEG is a simple, painless way to listen in on your brain's electrical activity. It is the cornerstone test for seizures and epilepsy and can help explain other symptoms such as fainting or confusion.

Clean, product-free hair and following your sleep and medicine instructions help make the recording as clear as possible. Bring your questions to the follow-up visit.

Keep learning: Read our epilepsy guide or our guide to functional MRI. For more background, see the MedlinePlus page on EEG.

FAQ

What is an EEG?

It is a painless test that records your brain's electrical activity through electrodes on your scalp. [1]

What does an EEG diagnose?

It mainly helps diagnose epilepsy and seizures and can look into confusion, fainting, memory loss or behavior changes. [1]

How long does an EEG take?

A routine EEG takes about 20 to 30 minutes; prolonged and ambulatory EEGs take an hour to several days. [1]

Does an EEG hurt?

No. The electrodes only record activity and do not cause pain. [1]

How should I wash my hair before an EEG?

Wash it the night before and skip conditioner and styling products. [1]

Do I need to stay awake before an EEG?

Sometimes. Your provider may ask you to stay awake the night before. [1]

What is an ambulatory EEG?

A portable recorder tracks your brain activity for one or more days while you go about your normal routine. [1]

What is a video EEG?

It records video and audio along with brain activity to show what happens during a seizure. [1]

Are there side effects of an EEG?

They are rare; dizziness from deep breathing or minor skin irritation can occur. [1]

What happens if my EEG is abnormal?

Your provider will discuss next steps and may refer you to a neurologist. [1]

Related Reading

Intraoperative MRI (iMRI): MRI Scans During Brain Surgery explains MRI scanning during surgery to check tumor removal.

Evoked Potential Test: VEP, BAER and SSEP for MS and Nerve Problems explains the nerve-signal tests that include VEP, BAER and SSEP.

Epilepsy: Seizures, Symptoms, Causes, and Treatment explains the condition an EEG is most often used to diagnose.

Grand Mal (Tonic-Clonic) Seizure: Complete Guide to Symptoms, Causes, Diagnosis, and Treatment covers a seizure type that often leads to an EEG.

Absence Seizures: What They Look Like, Who Gets Them, and How They Are Treated describes brief seizures that an EEG can capture.

Autoimmune Encephalitis: Complete Guide to Symptoms, Causes & Treatment explains a brain inflammation that EEG helps monitor.

Narcolepsy: Symptoms, Types, Causes, Diagnosis & Treatment — Complete Guide covers a sleep disorder evaluated with sleep studies and EEG.

Visual Evoked Potential (VEP) Test: Why It's Done, What to Expect and Results explains the eye-to-brain signal test used to detect optic nerve damage.

SEEG (Stereoelectroencephalography): Finding Where Seizures Start explains the deep-brain electrode test used to find where seizures start.

Magnetoencephalography (MEG): Mapping the Brain Before Epilepsy Surgery explains the magnetic brain scan used to find where seizures start.

References

1. Cleveland Clinic. Electroencephalogram (EEG): What It Is, Procedure & Results. Cleveland Clinic. Last updated September 29, 2025. https://my.clevelandclinic.org/health/diagnostics/9656-electroencephalogram-eeg

2. Krumholz A, Wiebe S, Gronseth GS, et al. Evidence-based guideline: Management of an unprovoked first seizure in adults: Report of the Guideline Development Subcommittee of the American Academy of Neurology and the American Epilepsy Society. Neurology. 2015;84(16):1705-1713. PMID 25901057. https://doi.org/10.1212/WNL.0000000000001487

3. Nurse ES, Freestone DR, Dabscheck G, et al. Clinical findings of long-term ambulatory video EEG following routine EEG. Epilepsy Behav. 2024;161:110104. PMID 39467452. https://doi.org/10.1016/j.yebeh.2024.110104

Medical disclaimer

This article is for general education only and does not replace advice from a qualified healthcare professional. Talk to your doctor about your own diagnosis and treatment, and seek emergency care right away for severe or sudden symptoms.