Reading Between the Lines – An Inside Look at the Polysomnogram

By Clinton Marquardt - Sleep & Fatigue Specialist

July 25, 2010

better sleep, sleep tests


This is an older generation polysomnograph. Back when it was used, over 900 pages of paper would be needed for one person’s recording. Today, polysomnograms are recorded using computers.

Has your doctor told you that you will have to get all wired up and then sleep in a lab while technicians and specialists watch you sleep? This is one way to find out what is going on with your sleep. It can be a little frightening and stressful so a little demystification of the process will be helpful.

When you sleep in the lab, the multitude of sensors and electrodes record brainwaves (electroencephalogram or EEG), eye movements (electroculogram or EOG) and muscle tone (electromyogram – EMG) in order to detect if you are awake or asleep. A heart rhythm strip (electrocardiogram or EKG or ECG) is also recorded as a precaution. If you are asleep, you may be in any one of the five different stages of sleep: 1, 2, 3, 4 & REM. You will also cycle in and out of the five stages every 90 minutes or so. The recording of your sleep is called a polysomnogram and it is recorded using a polysomnograph. The word means many (poly) sleep (somno) measurements (graph). For more information about over-night sleep studies visit Cleveland Clinic.

All of the stages of sleep have different patterns of brainwaves, eye movements and muscle tone. Each 30 seconds of recording is called an epoch and great examples of actual recordings are shown below.


Fast active brainwaves, elevated muscle tone, lots of eye movements and a steady heartbeat; these are the indications that you are awake.

Stage One Sleep

Stage one is the lightest stage. When you wake someone up from this stage they usually say that they weren’t sleeping. Brainwave activity begins to slow down, the eyes start rolling slowly, and muscle tone becomes very regular. These are a signs that you have fallen asleep.

Stage Two Sleep

Notice how the brainwave activity shows little spikes. These are called K-complexes and they are usually followed by spindle-like activity known as sleep spindles. Eye movement stops during stage two. You spend most of the night in this stage of sleep. Stages one and two are known as desynchronized sleep.

Stage Three Sleep

The deeper the sleep, the bigger and slower the brainwaves. Stages three and four are the deepest stages of sleep. Notice how similar the brainwaves and eye movements look? That’s because the eyes are so still and the brainwaves are so strong and synchronized that you can actually record brainwaves from the eye movement polysomnograph channel. It is during these stages when your brain is most at rest and under a condition of decreased blood flow.

Stage Four Sleep

This is the deepest stage of sleep. Most clinical sleep centres now group stage three and stage four sleep together and call it all stage three.  If you are researching sleep, stage four is still a useful category.  For stage four, notice lots of deep sleep brainwaves. Also look closely at the muscle tone and compare it with the muscle tone during REM sleep below. Sleepwalking occurs during stages three and four sleep. Do you know why they say you should never wake a sleepwalker? It’s because they are in such a deep stage of sleep that they can become disoriented, confused and violent when their brains move from deep sleep to wakefulness too quickly. The deep stages of sleep, three and four, are also known as delta sleep, slow wave sleep and synchronized sleep.

REM Sleep

Brainwave activity looks like it does when awake, it’s very active. Ah ha! We must be dreaming. Dreams are most vivid during REM sleep, although we do dream in the other stages of sleep. Notice the active Rapid Eye Movements (REM). Some theorists feel we may be watching what is going on in our dreams.

Meanwhile, muscle tone is completely flat during REM Sleep. That’s because we’re paralyzed so we don’t act out our dreams. The paralysis allows us to stay put in our beds. There is a disorder called REM Behaviour Disorder (RBD), where the person does not lose all muscle tone during REM sleep. These people are able to act out their dreams. Often these people will injure themselves or even hurt their bed partner.

Thanks to L. Orr, BA, RPSGT for her help with this article.

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