Non-rapid eye movement, 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 this stage. 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 acute and chronic schizophrenics 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).
Although study participants' reports of intense dream vividness during REM sleep and increased recollection of dreams occurring during that phase[clarification needed] suggest that dreaming most commonly occurs during that stage[clarification needed] , dreaming can also occur during NREM sleep.
Research has also shown that dreams during the NREM stage most commonly occur during the morning hours which is also the time period with the highest occurrence of REM sleep. This was found through a study involving subjects taking naps over specific intervals of time and being forcefully awakened, their sleep was separated into naps including only REM sleep and only NREM sleep using polysomnography. This implies that the polysomnographic occurrence of REM sleep is not required for dreaming. Rather, the actual mechanisms that create REM sleep cause changes to one's sleep experience. Through these changes, by morning, a sub-cortical activation occurs during NREM that is comparable to the type that occurs during REM. It is this sub-cortical activation that results in dreaming during the NREM stage during the morning hours.
Polysomnography (PSG) is a test used in the study of sleep; the test result is called a polysomnogram. Below are images of the NREM stages 1, 2 and 4 (prior to the merging of stages 3 and 4).
The figures represent 30-second epochs (30 seconds of data). They represent data from both eyes, chin, EEG, legs, microphone, intercostal EMG, sternocleidomastoid activity, nasal/oral air flow, thoracic effort, abdominal effort, EKG, oxymetry, and body position, in that order. EEG is highlighted by the red box. Sleep spindles in the stage 2 figure are underlined in red.
Stage 1: 
Stage 2: 
Stage 3: 
Slow-wave sleep (SWS) is made up of the deepest stage of NREM, and is often referred to as deep sleep.
The highest arousal thresholds (e.g. difficulty of awakening, such as by a sound of a particular volume) are observed in stage 3. A person will typically feel groggy when awoken from this stage, and indeed, cognitive tests administered after awakening from stage 3 indicate that mental performance is somewhat impaired for periods up to 30 minutes or so, relative to awakenings from other stages. This phenomenon has been called "sleep inertia."
After sleep deprivation there is usually a sharp rebound of SWS, suggesting there is a "need" for this stage.
Slow Wave Sleep (SWS) is a highly active state unlike a state of brain quiescence as previously thought. Brain imaging data has shown that during nonREM sleep the regional brain activity is influenced by the waking experience just passed.
A study was done involving an experimental and a control group to have them learn to navigate a 3D maze. The blood flow in the parahippocampal gyrus increased in conjunction with the individual's performance through the 3D maze. Participants were then trained in the maze for 4 hours and later, during the various sleep cycles of nonREM sleep, REM sleep and wakefulness, they were scanned twelve times using PET during the night. The PET scan demonstrated a higher blood flow in the hippocampus during SWS/non-REM sleep due to the training from the previous day while the control group exhibited no increased blood flow and they had not received the training the prior day. The brain activity during sleep, according to this study, would show the events of the previous day do make a difference. One theory suggests a model of Hippocampal-neocortical dialogue. "Two stages of hippocampal activity have been proposed, the first being the recording of the memory during waking and the second involving the playback of the memory during nonREM sleep. This process of reactivation of memory firing sequences is believed to gradually reinforce initially weak connections between neocortical sites allowing the original information to be activated in the cortex independently of the hippocampus, and thus ensuring refreshed encoding capacity of the hippocampus." Maquet concluded that the areas of the brain involved with information processing and memory have increased brain activity during the slow wave sleep period. Events experienced in the previous day have more efficient and clearer memory recall the next day thus indicating that the memory regions of the brain are activated during SWS/non-REM sleep instead of being dormant as previously thought.
"sleep." Encyclopædia Britannica. Encyclopædia Britannica Online. Encyclopædia Britannica Inc., 2011. Web. 05 Dec. 2011.<http://www.britannica.com/EBchecked/topic/548545/sleep>.
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