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.
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