To appreciate the function of normal sleep and how various sleep disorders occur, it is necessary to have an understanding of the basics of sleep stages.
The various sleep stages are distinctive, and they can be identified through the use of a limited EEG. The EEG, which is performed during a standard sleep study (polysomnogram), is a measurement of the continuous brain wave patterns, or electrical activity of the brain.
The recorded activity can be categorized into two basic stages -- non rapid eye movement (NREM) and rapid eye movement (REM) sleep.
What Is NREM Sleep?
Non rapid eye movement (NREM) sleep consists of three stages (N1, N2, and N3), and each has recognizable electrical brain wave patterns. NREM makes up the largest portion of the sleep cycle.
NREM is characterized by decreased blood flow to the brain and skeletal muscle. There is also decreased heart rate, blood pressure, and volume of air moving in and out of the lungs.
Stage 1 (N1) involves slow rolling eye movements and partial relaxation of voluntary muscles.
Stage 2 (N2) shows characteristic patterns on the EEG, including K complexes and sleep spindles. The K complex is a high amplitude wave, meaning that it is tall and wide. Sleep spindles are closely spaced high frequency waves, meaning that they are not as tall and occur quickly over a short period of time. These are recognized on sleep studies by trained individuals and are used to identify sleep stages.
Stage 3 (N3) shows high amplitude activity consisting of tall and wide waves on EEG and is recognized as the deepest of the three NREM stages of sleep.
What Is REM Sleep?
Rapid eye movement (REM) occurs several times during sleep, but it comprises the smallest portion of your sleep cycle. It is notable for a presence of rapid eye movement (REM), which is the movement of your eyes in different directions while you are asleep. This is a deep stage of sleep with intense activity in certain parts of the brain. The EEG recordings, more patterned in earlier stages, become desynchronized during REM.
This is the stage of sleep when dreaming occurs.
With the exception of your eye muscles and diaphragm, you have no motor function during REM. This loss of muscle tone during REM sleep may conserve energy and protect you from acting out your dreams.
In addition, REM leads to changes in blood pressure, heart rate and breathing rate. Blood flow is increased to the brain, as well as the penis and clitoris, resulting in engorgement.
Dividing Sleep into Patterns
Sleep architecture represents the structure of your sleep and is generally composed of a somewhat cyclical pattern of the various NREM and REM sleep stages.
In general, there are four to six cycles of NREM sleep per night, which are followed by brief intervals of REM sleep. Each cycle lasts about 90 minutes. As the night progresses, the periods of NREM become shorter and the periods of REM become longer. The average adult will spend about 20% to 25% of the night in REM sleep, but this may decrease as we age.
Mowzoon, N et al. "Neurology of Sleep Disorders." Neurology Board Review: An Illustrated Guide. 2007; 720-722.