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Sleep being an essential component of life oscillates through two main categories viz. the rapid eye movement (REM) and the non-rapid eye movement (NREM) sleep stages. These two major sleep stages are further divided into five stages through which a person drifts in the course of one night.
The study of sleep largely depends on the electrical activities of the brain usually determined by measuring the electrical changes in the brain by using electroencephalograms (EEGS). Furthermore, voltage generated by eye rotation in their sockets and electrical activities of the muscles all help in the study of the cycles of sleep in the course of one night.
Sleep cycles in a health adult person
The recording of brain waves, eye movements, and the muscle tension has led to the categorization of sleep stages into five stages. The non rapid eye movement generates four of the stages while the rapid eye movement constitutes the fifth stage.
According to Ricker a psychologist, “when we first fall to sleep, we are in stage 1, from there we progress through stages 2, 3, and 4, and then, after about an hour enter a stage of rapid eye movement sleep” (2010, p.32). Each of these stages experience specific behavioural and mental activities as well as electrical brain changes.
In stage 1 of sleep, commonly known as the light sleep, a person is in transition between wake state and sleep state. The brain at this stage is characterised by high influx of the beta waves. In addition, at this stage, a person awakes out of mere distraction such as simple noise.
Study shows that when one awakes out of light sleep stage, often s/he does not know that s/he was actually asleep. Ricker also observes that, “the light sleep stage episode lasts for about five minutes and makes approximately 2 to 5% of the total sleep in a night” (2010, p 34). Stage 1 then heralds stage two in the five-stage sequence.
Stage 2 of sleep, also known as the moderately light sleep stage as studied by Maquet , Stickgold and Smith, is characterized by and bears the characteristics of “rapid electrical activities of the brain and large fluctuation of voltage known as the sleep spindles and k-complexes respectively” (2003, p.86).
These electrical activities and changing voltage creates high peaks and deep valleys in the brain waves as created in the electroencephalogram, which shows less active cerebral cortex, and this reduces further as one gets deeper into sleep. The episode of the moderate light sleep lasts for about 10 to 25 minutes and account for relatively 45 to 55% of the total sleep in one night of a health adult person.
Stages 3 and 4 are closely connected but they are distinct with stage 3 acting as a transitional stage lasting for about five minutes. Stage 3 involves an increase of the delta waves to the brain while on the other hand cerebral cortex activity reduces gradually. Ricker observes that, “people become harder to awaken” (2010, p. 34).
The major difference between stages 3 and 4 lies in the amount of the delta waves increase. When delta waves accumulate to about 50% in the brain, the person drifts from stage 3 (transitional sleep) to stage four (deep sleep). Ricker in his study says, “The episode of deep sleep lasts for about 20 to 40 minutes. Both stage 3 and 4 combined make up 2 to 20% of total sleep in one night” (2010, p.36).
From deep sleep, a person drifts to the rapid eye movement sleep characterised by features of an awake person; for instance, beta waves usually seen when a person is awake and alert, blood pressure and heart rate and decreased muscle tone.
Interestingly, at this stage a person starts dreaming. A great percentage of people who wake up at this stage admit having been dreaming. According to Ricker, “when sleeper are awaken from the rapid eye movement sleep, about 89% report that they were just dreaming” (2010, p.36). Electrical activities in the rapid eye movement sleep are faster and random as compared to those in non rapid eye movement sleep stages.
These five stages of sleep occur systematically in a regular format in the course of sleep during every night when one goes to sleep. One complete cycle comprises of the five distinct stages occurring sequentially and a health adult person normally experiences about six cycles in the course of sleep in one night. Each cycle lasts for approximately 90 minutes.
During sleep, a healthy adult person undergoes five stages of sleep cycle with each stage containing distinct characteristics triggered by the brain’s electrical activities. The cycles of sleep comprise the five stages of sleep progressing from stage1 through to stage four and finally one enters the rapid eye movement sleep characterised by dreaming.
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Each stage of sleep accounts for a particular percentage of the total sleep and lasts for a specific duration in the course of each cycle. In some stages, a person drifts from sleep to wake state, and therefore easily awakens by slight physical stimulus while on the other hand, in other stages such as deep sleep and the rapid eye movement sleep, it remains hard to awaken an individual.
Maquet, P., Smith, C., & Stickgold, R. (2003). Sleep and brain plasticity. New York: Oxford University press.
Ricker, J. (2010). Introduction to psychology. New York: William morrow press.