The Sleep process is a physiological behavior, that is common to all animals. It accounts for one-third of human life. It is not clearly known what the exact functions of sleep are but it seems to be essential for survival because prolonged sleep deprivation leads to severe physical impairment followed by cognitive loss and eventually death.
Sleep disturbances are clinically important because they are indicators of almost all types of psychiatric disorders.
1-Definition of sleep
Sleep is a state where awareness of factors in the environment is reduced. Sleep differs from states of coma, hibernation, and death in the fact that it can be rapidly reversed.
2-Phases of sleep
Like other mammals, humans also exhibit two types of sleep:-
rapid eye movement (REM)
Rapid eye movement (NREM) sleep
These two types of sleep have distinct characteristics when assessed using an electroencephalogram or EEG.
EEG monitors brain activity and electrical impulses within the brain during wakefulness and sleep. REM and NREM sleep are also slightly different from each other in terms of physical properties.
For example, there are repeated bursts of eye movement activity during REM sleep. The EEG during REM sleep shows similar patterns as when the person is awake. Thus, it is a paradoxical state of sleep when the brain appears awake on the EEG but the individual is in deep sleep.
EEG activation is reduced during NREM sleep. It is also called orthodox sleep. The mind seems calm.
3-Stages of sleep
Sleep is divided into stages. Each stage is almost distinct from the sleep cycle. These cycles are repeated more than once each night while the person sleeps.
Three parameters are used to evaluate the phases and stages of sleep visual scoring. These include:-
E-E-G - records the electrical activities of the brain.
Electrooculogram (EOG) – This device records eye movements.
Electromyogram (EMG) – This records muscle tone and movement while the person sleeps. Records are made with testing under the chin.
When a person is awake, there is a low-voltage rapid activity or activated pattern on the EEG. Eye movements and blinking occur despite closed eyes. These eye movements are voluntary. EMG shows a high muscle tone.
EEG shows alpha activity eight-thirteen cycles per second.
4-Four stages of sleep
Once sleep occurs, there are four stages of sleep. Stage 1 sleep is the initial NREM stage. Here the alpha activity ends. The pattern changes to a low-voltage mixed-frequency EEG pattern and the EEG has prominent theta activity (3-7 cps) with fast waves. Eye movement slows down and muscle,s tone decrease.
During stage 1 the individual is aware of their surroundings and can be easily awakened. Sudden muscle cramps and a feeling of falling or falling asleep.
Stage 1 lasts for a few minutes before progressing into Stage 2. Here the E-E-G shows the appearance of sleep spindles {12-14 cps} and K-complexes. These K complexes are high-amplitude negative fast waves followed by positive slow waves. This is also an N-R-E-M stage and the beginning of sleeping as the person perceives it.
Step no 2 is followed by steps 3 and 4 accordingly. E-E-G shows slow waves {<2 cps in humans}. Stages 3 and 4 are also called slow wave sleep (SWS), delta sleep, or deep sleep. Motivation is difficult at this stage. As a person progresses from stages 2 to 4, eye movements decrease. EMG shows the contraction tone of the muscle.
NREM sleep stages 1 through 4 are followed by REM sleep. The EEG during REM sleep looks similar to stage 1 NREM sleep. Theta waves (3-7 cps) and loss of muscle tone occur. Eye twitching and muscle spasms may occur.
5-Duration of sleep
The amount of sleep varies from person to person. A normal sleep requirement is about 7-9 hours of sleep per night. Some people can function well with less than 6 hours per night while others may need around 12 hours of sleep.
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