How do you measure sleep?
This is usually done in a sleep laboratory or a sleep center. Technicians attach electrodes to the head to take three types of measurement.
First, electrical activity in the brain is measured by electroencephalography (EEG). This measure is used because the EEG signals associated with being awake are different from those found during sleep. Also, the different stages of sleep can be measured using EEG.
Second, muscle activity is measured using electromyography (EMG), because muscle tone also differs between wakefulness and sleep. Once again, there are EMG differences within sleep, depending upon the stage of sleep.
Third, eye movements during sleep are measured using electro-oculography (EOG). This is a very specific measurement that helps to identify Rapid Eye Movement or REM sleep, during which we often dream. The eyeballs make characteristic movements that show us when someone is in this type of sleep.
This whole system of assessment is usually called polysomnography (PSG). The prefix 'poly' simply refers to the fact that more than one type of physiological activity is being measured.