Dream Stages

The various stages of sleep were described for the first time in the 1930s by Loomis, and later in the 1950s by Aserinsky and Leitman, identified Rapid Eye Movement (REM) sleep. 

NREM and REM

Sleep can be divided into two main categories:

Non-rapid Eye Movement (NREM) sleep
Rapid Eye Movement (REM) sleep

Sleep begins with NREM or SWS (slow wave sleep) and progresses to REM sleep. NREM and REM occur in alternating cycles, the duration of each cycle spanning about 90-100 minutes, with a total of 4 to 6 cycles. There are three major slow oscillatory rhythms in NREM sleep and one fast oscillating rhythm during waking and REM.

NREM (Non-rapid Eye Movement)

Stage 1 occurs in the beginning of sleep with slow eye movements. During this stage the alpha waves disappear and theta waves appear.  During the period of transition into Stage 1 NREM, people experience hypnic jerks.  People woken up at this stage believe that they were fully awake. 

Stage 2 is an unconscious stage but the sleeper is easily awakened during this stage.  No eye movements occur during this stage therefore dreaming is of rare occurrence during this stage of sleep. EEG recordings show characteristic sleep spindles and K-complexes during this stage.  This rhythm is generated by the reticular nucleus of the thalamus.  The spindle stage blocks transmission of the outside environment with the cortex.  Though neither the cortex nor the thalamus require the other for function during these stages they do interact.

Stage 3 is the transition between stage 2 and stage 4 where delta waves associated with deep sleep start occurring.

Stage 4 is the slow wave sleep stage, also referred to as SWS.  This happens to be the deepest stage of sleep where there is continuation of delta waves.  Dreaming is most common during this stage when compared to the other stages of NREM sleep but not as common as in REM sleep.  The contents of SWS dreams tend to be disconnected, less clear, and less memorable when compared to the content of dreams that occur during REM sleep.  This is also the stage in which parasomnias most commonly occur.

REM (Rapid Eye Movement)

This is a normal stage of sleep characterized by rapid movements of the eyes.  REM sleep can be further classified into two main categories; tonic and phasic, which was first identified and defined by Kleitman and Aserinsky in the early 1950s.

REM sleep in a normal healthy young adult takes up about 20-25% of total sleep, which translates to about 90 to 120 minutes of a night’s sleep.  During the course of a normal night of sleep, humans usually experience about 4 to 5 periods of REM sleep; they are relatively short at the beginning of the night and longer towards the end.  Most people tend to wake, or experience a period of very light sleep, for a short time immediately after a bout of REM.  The relative amount of REM sleep varies considerably with age.  A newborn baby spends more than 80% of total sleep time in REM.

During REM, the activity of the brain’s neurons is similar to that during waking hours, for this particular reason, the phenomenon is called paradoxical sleep, which means that there are no dominating brain waves during REM sleep.

REM sleep is physiologically different from the other phases of sleep, which are collectively called non-REM sleep.  Vividly recalled dreams mostly occur during REM sleep.  These dreams are also those dreams that most likely to be remembered upon waking up.  It is felt that that the length of the REM sleep cycle might be correlated to the brain size. 

Hobson’s reciprocal-interaction model gives a biological explanation to the chemical transaction that take place during REM. REM is generated in the pontine reticular formation and REM is turned on by cholinergic neurotransmitters. Serotonergic and noradrenergic neurotransmitters inhibit the cholinergic neurotransmitters turning REM off. The function of REM has not been definitely determined. One theory is an evolutionary, adaptive function. Another function may be a restorative process for some types of memory during REM sleep, but it has been firmly established that REM is when dreaming occurs in the sleep cycle.

Once it was established that dreams are generated during REM, it became very important to find out the reasons for dreaming and to get into the details of the functions of dreaming.  There are two fundamental theories that describe the function of dreaming.  One is an adaptive evolutionary function and another is restorative memory function.

While psychoanalysts argued that we dream to forget and not include certain traumatic memories into our dreams, neurobiologists insisted that dreaming is a phenomenon during which suppressed and repressed memories are brought to the surface. Neurobiologists concluded that disturbances in REM sleep end up in nightmares and horrifying dreams; which could also result in narcolepsy because of an increase in REM latency and a decrease in REM time.  Hence they felt that people who have gone through violent experiences try not to allow these experiences to be part of their long term memory and that the entire function of dreaming was a step towards memory consolidation.

Current research suggests a more involved chemical exchange in the reciprocal-interaction model than was once thought. Though dreams may not have meaning as Freud suggested, dreams still seem to reflect an individual’s stored memory and emotion brought about by chemical exchanges in the brain and brainstem.

Features of Dreams

What are the predominant features of dreaming and how do they occur?  To begin with there are four features of dreams, and they are bizarreness, motor hallucination, emotion, and memory deficit. 

Bizarreness is described as being the incongruence of plot, the instability of time, people, and places. Bizarreness consists of various aspects of activation and deactivation within the brain. The deactivation of the fusiform gyrus, is responsible for face recognition. The deactivation of the parietal operculum is responsible for special imagery. The dorsolateral prefrontal areas and precuneus both associated with the encoding and retrieving of episodic memory. The prefrontal area and the parietal lobules are associated with working memory.

Motor hallucinations, visual and motor, are active behaviors such as running, flying etc,. Emotions are perceived as an increased intensified fear, anxiety and anger. Memory deficits make recall during consciousness difficult.  The causes of these features are the activation or deactivation of certain structures within the brain and brainstem. Motor hallucinations may be caused by the activation of the basal ganglia, visual cortex, lateral geniculation nucleus motor cortex and auditory cortex.

Emotions are caused by the activation of medial limbic and paralimbic structures, which shape the plot of the dreams through the emotions and not vise versa.

Memory deficits are thought to be the absence of noradrenaline and serotonin, which are crucial to learning and memory.

Excerpts taken from this article are licensed under the GNU Free Documentation License. They use material from Wikipedia topics "Lucid Dream" and/or "Sleep".