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Taylor Evans

The Rhythm of Rest: Understanding Sleep Cycles on an EEG


As EEG Technologists, we love sleep, not just for ourselves but for our patients. We love to see it and encourage it. Sleep is essential to see on an EEG.

 

When a patient begins the EEG, they are typically fully awake. As the study begins, we prioritize their comfort, intending to have the patient fall asleep. We create a relaxing environment by dimming the lights and encouraging them to close their eyes, signaling the start of stage 1 sleep.



 

During a routine outpatient EEG, we typically see stages of non-REM sleep: N1, N2, and N3. Because of the length of our studies, we rarely see REM sleep. However, we may see REM on extended EEGs.

 


1.        The stages of non-REM sleep :

 

Stage 1 (N1): The alpha rhythm will slow down to theta waves (4-7 Hz), and we will start seeing rolling eye movements in the frontal leads. This stage is also called drowsiness and is the earliest stage of sleep. A true mark of the onset of N1 sleep is the appearance of vertex waves. Vertex waves are normal bilateral phase reversing discharges best seen over the central regions.

 



Stage 2 (N2): In stage 2 sleep, we will see continued theta waves with occasional vertex waves and an appearance of sleep spindles and k-complexes.  Sleep spindles are characterized by short bursts of high-frequency waves at about 12-16 Hz. We will predominately see these in the central leads. K-complexes consist of a brief negative high-voltage peak followed by a slower positive complex, and bursts of sleep spindles often follow them. K-complexes can be a response to an internal stimulus, like a respiratory interruption, or an external stimulus, like a knock on the door or being touched.

 

 


Stage 3 (N3): Stage N3 of sleep is characterized by high-amplitude delta slowing in the range of 0.5- 2Hz. This stage is also known as slow-wave sleep (SWS). As patients are in a deeper stage of NREM sleep, we will also see fewer muscle artifacts and less eye movement in this stage. Stage N3 sleep occurs most frequently at night, and it's harder to awaken someone from this stage.




 



2.         Rapid Eye Movement Sleep.


REM Sleep ( Rapid Eye Movement Sleep): REM sleep can be distinguished by the sawtooth pattern in the theta range (4-8 Hz) on an EEG. The pattern of REM sleep on an EEG can look like the awake state, with fast desynchronized rhythms. REM sleep is associated with dreaming, and people can repeat this stage 4-5 times a night, with each sleep cycle lasting 90-110 minutes.  Routine EEGs are typically 30 minutes, so we do not see the complete sleep cycle.  The study would need to be extended to see REM sleep in an EEG, but if abnormalities were suspected, a patient would likely need a sleep study.

 

 




 

It is crucial to get the stages of sleep during an EEG because we may be able to see some information on the EEG that is not available in the awake state. As patients drift into the different stages of sleep, we can see other patterns and possible abnormalities.

 

 

 

 Taylor Evans, R. EEG T.

 



 References:



Gandhi MH, Emmady PD. Physiology, K Complex. [Updated 2023 May 1]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK557469/

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