What is a PSG? The PSG is actually a group of three different tests performed by different physicians, all conjoined to create a sleep test. The first part, and possibly the most important, is the EEG or electroencephalogram. This test is normally used by neurologists to measure brain wave frequencies that can differentiate such conditions as epilepsy, and Alzheimer’s disease. Happily enough for sleep physicians, each stage of sleep has its own unique wave form that can not only differentiate sleep from wakefulness, but also differentiate each component of sleep.
Sleep is divided into 2 states, REM (rapid eye movement) and NREM (non-rapid eye movement). The NREM state is further subdivided into 3 distinct stages. The first stage is characterized by high frequency low amplitude waves called alpha waves in which one is resting quietly with his/her eyes closed. This sleep is also called light sleep and is seen at sleep onset. The second stage of sleep is characterized by sleep spindles and k-complexes. The frequency is slightly slower and the amplitude of the waves is slightly larger. The final stage of NREM sleep is stage 3. This stage of sleep is characterized by slow wave sleep with very high voltage waves. This stage of sleep is deep sleep and is also known as “restful sleep.” REM is characterized by rapid eye movement with muscle paralysis and a “saw-toothing” (continuous peaking bursts on EEG voltage) EEG pattern.
The other components of the PSG are the EOG (electro-oculogram) and the EMG (electro-myogram). The first EOG is used to evaluate the eye movement during each phase, especially REM sleep. The first stage of sleep also has a slow eye rolling pattern that is discernable and can be detected by EOG. The EMG is used to evaluate muscle tone throughout the night. This tool can help in determining any paralysis throughout the night that can help detect REM sleep, or any periodic limb movements (PLMs) that could cause arousals during the night.
Sleep and its staging is a process that has been studied for years by board certified physicians. However, this introductory lesson should be enough to hold an intelligent conversation if ever asked about the process! Also, this summary can help dispel any rumors about sleep studies as they are becoming more and more prevalent in today’s society, and patients want to understand what they are having done.