Before we launch into a discussion about the different types of sleep, it is important to first note the different measurements that are used to study sleep.
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 data in the form of 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 often depicted side by side when data is presented.
Brain Activity – EEG
The electroencephalogram (or EEG) is a measurement of bioelectric brain activity. It is generated through electrodes placed on the scalp and is vital for studying sleep. When looking at EEG levels sleep researchers can clearly see periods such as sleep onset and the different stages of sleep. They can determine what they are by looking at the shapes of the polygraph.
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:
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.