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.
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
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
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.