SLEEP AND ITS STAGES
Sleep is a behavior, although muscular movements appear to be involved. Movements that do occur in sleep are limited to a particular stage. Sleep is an urge, a need with which we must comply. But we remember very little about what happens while we sleep. There is a change in consciousness.
Biological Rhythm.
Much of our behavior follows regular rhythms. Daily rhythms in behavior and physiological processes are found throughout the plant and animal world. We have a number of natural cycles, which form part of our everyday lives. Some of these operate over several weeks, such as the human menstrual cycle, while others are much more rapid, such as the human menstrual cycle, while others are much more rapid, such as the cycles involved in digestion and food intake. Some rhythms, called circadian rhythms, are based on the 24hours of the day, and perhaps the must obvious circadian rhythm is the sleep-wake circle we alternate throughout life, between periods of wakefulness and periods of sleeping, human beings, for the most part tend to be awake during the day and sleep at night-they are diurnal animals. Other animals are awake at night and sleep during the day, they are known as nocturnal animals.
Sleep Rhythms
Research on human sleep is usually conducted in a sleep laboratory such laboratories are equipped with machines for taking electrophysiological measurements. Electrodes placed on the scalp monitor the electroencephalogram gram (EEG): those attached to the chain monitor muscle activity, which are recorded as the electromyogram (EMG) electrodes attached around the eyes monitor eye movements, recorded as electroculogram (EOG) in addition, other electrodes can be used to monitor autonomic measures such as heart rate electrocardiogram - EKG or ECG respiration electrostethogram-ESC and skin conductance (or galvanic skin response-GSR).
During wakefulness. The EEG of a normal person shows two basic patterns of activity: alpha activity and beta activity. Alpha activity consists of regular, medium frequency waves of 8-12 Hz. the brain produces this activity when a person is resting quietly, not particularly aroused or excited and not engaged in strenuous mental activity. Alpha waves occur most often when the eyes are closed. Beta activity consists of irregular, mostly low-amplitude waves of 13-30Hz. This activity occurs when a person is alert and attentive to event in the environment or is thinking activity.
During the course of the night, a normal person pass through four different stages (or levels) of sleep several times. Moving from higher to lower levels of sleep, and back to higher levels again. This cycle is repeated several times during a typical night although the deeper levels of sleep tend to be reached only during the first few cycles. Towards the end of the night sleep cycles become shallower and the person is liable to wakeup easily. These changes in EEG pattern reflect how deeply the person is asleep.
Stage 1 sleep is marked by the presence of some theta activity (3.5-7.5Hz) the level is actually a transition between sleep and wakefulness. It lasts for about ten minutes. It is the lightest level of sleep from which one can be very easily awakened. Breathing is irregular and the muscles begin to relax as the person transit into the next stage.
Stage 2. the EEG during this stage is generally irregular but contains periods of theta activity sleep spindles and K complexes. Sleep spindles are short burst of waves of about 12-14 Hz that occur between two and five times during stages 1-4 of sleep. They presumably represent the activity of a mechanism that decrease the brain's sensitivity to sensory input and thus keep the person asleep. The sleep of older people contain fewer sleep spindles and is generally accompanied by more awakenings during the night. K complexes are sudden. Sharp waveforms, which unlike sleep spindles, are usually found only during stage 2 sleep. They spontaneously occur at the rate of approximately one per minute but are often can be triggered by noise. They represent mechanisms involved in keeping the person asleep. Stage 2 sleep lasts for about 15 minutes and is a deeper level of sleep. Persons woken up during this level of sleep will deny having been asleep.
Stage 3 sleep is marked by the occurrence of high-amplitude delta activity (less than 3.5 Hz). this is a transition from the light sleep of stages 1 and 2 to a deeper sleep. The distinction between stage 3 and stage 4 is not clear-cut; stage 3 contains 20-50 percent delta activity: and stage 4 contains more than 50 percent. Stage 3 sleep last for about 20 minutes.
Stage 4 sleep is the deepest level of sleep from which it is difficult to awaken the sleeper. It is characterized; also, by delta activity (EEG readings of less than 2.5 Hz) usually takes more than half an hour reach this level of sleep. About 90 minutes after the beginning of sleep (and about 45 minutes after the onset of stage 4 sleep) they will be an abrupt change in a number of the physiological measures being taken. The EEG recording become desynchronized (i.e. irregular, with a sprinkling of theta waves. Very similar to the record obtained during stage 1 sleep. The EOG will record that the eyes are rapidly darting back and forth beneath the eye, the EMG becomes silent due to a profound loss of muscle tonus. Thus apart from an occasional twitching of some muscles, the person is actually paralyzed during this period. This peculiar stage of sleep is quite distinct from the quiet sleep we saw earlier. This type of sleep is usually referred to as REM or rapid eye movement sleep. It has also been called paradoxical sleep because of the presence of beta activity. This is usually seen during wakefulness or stage sleep.
REM Sleep Stages 1-4 are usually referred to as non-REM or quiet sleep (QS). Stages 3 and 4 are referred to as slow-wave sleep or deep quiet sleep (DQS): while stages 1 and 2 are known as light quiet sleep (LQS).
On the average most normal persons have four or five periods of active sleep at night. Each of these cycles last for about 90 minutes, containing a 20- 30 minutes about of REM as sleep.
