NREM sleep

Non-rapid eye movement sleep, or NREM, is, collectively, sleep stages 1–3, previously known as stages 1–4. Rapid eye movement sleep (REM) is not included. There are distinct electroencephalographic and other characteristics seen in each stage. Unlike REM sleep, there is usually little or no eye movement during these stages. Dreaming is rare during NREM sleep, and muscles are not paralyzed as in REM sleep. People who do not go through the sleeping stages properly get stuck in NREM sleep, and because muscles are not paralyzed a person may be able to sleepwalk. According to studies, the mental activity that takes place during NREM sleep is believed to be thought-like, whereas REM sleep includes hallucinatory and bizarre content. The mental activity that occurs in NREM and REM sleep is a result of two different generators, which also explains the difference in mental activity. In addition, there is a parasympathetic dominance during NREM. During the period of Non-REM sleep, the mindset of a person is more organized. The differences in the REM and NREM activity reported is believed to arise from differences in the memory stages that happen during the two methods of sleep. It has been found through several experiments that low levels of stage 3 sleep are found in about 40-50% of people with acute and chronic schizophrenia who typically portray abnormal non-rapid eye movement sleep.

NREM sleep was divided into four stages in the Rechtschaffen and Kales (R&K) standardization of 1968. That has been reduced to three in the 2007 update by The American Academy of Sleep Medicine (AASM).

Sleep spindles are unique to NREM sleep. The most spindle activity occurs at the beginning and the end of NREM. Sleep spindles involve activation in the brain in the areas of the thalamus, anterior cingulate and insular cortices, and the superior temporal gyri. They have different lengths. There are slow spindles in the range of 11 – 13 Hz that are associated with increased activity in the superior frontal gyrus, and fast spindles in the range of 13 – 15 Hz that are associated with recruitment of sensorimotor processing cortical regions, as well as recruitment of the mesial frontal cortex and hippocampus. There is no clear answer as to what these sleep spindles mean, but ongoing research hopes to illuminate their function.

K-complexes are single long delta waves that last for only a second. They are also unique to NREM sleep. They appear spontaneously across the early stages, usually in the second stage, much like the sleep spindles. However, unlike sleep spindles, they can be voluntarily induced by transient noises such as a knock at the door. The function of these K-complexes is unknown and further research needs to be conducted.

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