Written by Kevin Morton with adaptations from the Stanford Sleep Book

Before you launch into your journey to learn all about sleep, it is important to first understand the different measurements that are used to study and make distinctions in the land of slumber.

Dr. Dement applies electrodes to his wife Pat to perform some of the earliest all-night, continuous EEG recordings.


The standard method used to study sleep is called polysomnography. It uses electrode patches placed on specific parts of the sleeper’s head and body to record electrical activity on a polygraph (resulting in readable data that looks like scribbled lines).

There are a lot of places on the body that sleep researchers can measure activity from, but eye movements, muscle movements, and brain activity are especially important. These three target spots are known as the polysomnography core, and are often depicted one on top of the other when sleep data is presented.

Brain Activity – EEG

For instance, the presence of a sleep spindle (depicted above) signifies that the sleeper is in non-REM sleep, stages 2-4.

The electroencephalogram (or EEG) is a measurement of bioelectric brain activity (The Greek word enkephalos means “brain,” so “electro-encephalo-graphy” signifies graphing the electrical activity of the brain). EEG measurements are generated through electrodes placed on the scalp and have proven a vital tool for studying sleep.

When looking at EEG levels, sleep researchers can clearly see distinctions between the different stages of sleep, as well as when the initial sleep onset actually occurs. By looking at the shapes of the polygraph lines, they can better understand the sleeper’s journey through sleep.

Eye Movements – EOG

Electro-oculography (or EOG), the standard method used to measure eye movements during sleep, is incredibly useful for studying rapid eye movement sleep (which you’ll learn about in Sleep Step 2).

It’s also really cool how EOG works. It was a breakthrough for sleep scientists when it was discovered that the front of the eye (the cornea) is electrically positive compared to the back (the retina). Therefore, when the eye moves left, right, up, or down the change in voltage can be picked up by electrodes placed on the sleeper’s face and recorded on the polygraph.

Here’s a picture of the electrical charges of the eye to give you a sense of how this works:

Muscle Activity – EMG

The third thing that is virtually always measured when studying sleep is muscle activity, by a method called the electromyogram (or EMG).

Muscles emit electrical potentials when they move, and that electricity is also detected by electrodes and recorded on the polygraph. EMG electrodes are usually placed over the sleeper’s chin muscles.

The Polysomnography Core In Action

This image shows the standard electrode placement sites used in polysomnography, in addition to some typical polygraph readings to the right. The numbers indicate locations that are part of the more complex, standardized system that is used around the world. In non-sleep clinical tests that attempt to detect abnormal brain waves, as many as 20 or more electrodes are used on the scalp. In routine sleep studies, however, only one EEG tracing is necessary.

Although it is possible to measure a large variety of physiological processes during sleep, typically not more than three additional measurements are taken–breathing, heart rate, and the contraction of leg muscles.

These recordings serve crucial purposes in diagnosing and treating patients in sleep disorders centers, where the cumulative total of all the measurements are bunched together into the term clinical polysomnography.