As the name suggests, polysomnography measures many different aspects of sleep at one time (poly=many, somno=sleep, graphy=writing or record). There are multiple recording devices used to document various aspects of sleep in polysomnography. The electroencephalograph (EEG) measures electrical brain activity, commonly known as brainwaves. The electrocardiograph (ECG) measures electrical activity in the heart muscle. The electromyograph (EMG) measures muscle tone and activity. The electrooculograph (EOG) records eye movements. Respiratory rate and blood oxygen levels are also measured in the polysomnography process. Usually, a video recording of the sleeper is also made. There is an abbreviated version of the full polysomnography process that can be administered at home, but it is not usually recommended because it is not as complete, and because a technician does not remain through the night to ensure proper recording.

A full polysomnogram (the printed record of all the measurements taken) requires the attachment of about 16 - 17 electrodes to the scalp, face, chest, and legs. There are usually 6 electrodes used to record the EEG, 2 - 3 for the ECG, 6 for the EMG, and 2 for the EOC. Data is recorded from the pre-sleep awake state through all sleep cycles to the post-sleep awake state.

By examing all aspects of the polysomnogram, technicians and medical specialists can detect signs of sleep disorders, and determine their severity, while researchers can learn about normal sleep physiology. In fact, most of what we know about sleep physiology is the result of polysomnographic studies.