Pale Rider
11-08-2007, 10:44 AM
What is Sleep?
All of us know what sleep is. We know how it feels. The meaning of the word is generally not questioned in ordinary conversation, and we do not have to look it up in the dictionary.
But no one knows what sleep really is. Once we get beyond the simple level of recognizing a superficially quiet state that is very different from wakefulness, we find that the major questions about the nature of sleep are largely unanswered. These questions are:
What happens to us while we sleep? At a time before sleep was studied scientifically, laymen and intellectuals alike thought they knew the answer, i.e.., that sleep is a state in which "everything simply slowed down or stopped." That answer is wrong. Although our bodies move less and we make less noise while we sleep, there are many complex changes in our brains and minds during sleep, which cannot be described as a simple reduction of activity. For example, there are nerve cells in our brains, which fire 5 to 10 times more frequently during certain sleep stages than during wakefulness.
Why do we sleep? For millennia, mankind thought it knew the answer. If sleep is a state of inactivity, then it must be for rest. That answer is probably wrong. A few sleep scientists still believe that is true, but most doubt it. More activity during wakefulness does not reliably produce more sleep, and physical rest does not remove the urge to sleep, if anything, it increases the urge. Most answers to the question of why we sleep are theories, not facts.
What are the mechanisms of sleep? So long as we believed that sleep was a state of inactivity that we needed for rest, there was no need to worry about the mechanisms of sleep, about how it got started and was maintained. The apparent answer was that when our bodies and brains got tired, they simply stopped working. That answer is wrong! Although we do not know all the answers about how sleep works, scientists now know that it doesn't occur passively because nothing else is happening. There are mechanisms in the brain that actively induce and maintain sleep. For example, when certain specific parts of the brain are destroyed or inactivated by chemicals, which inhibit nerve action, sleep can be reduced or certain sleep stages can be eliminated entirely.
Thus, our concept of the essential nature of sleep has evolved over the past 35 years from an intuitively appealing, but erroneous, belief that sleep is simply a state of inactivity which occurs passively when organs became fatigued in order they might rest, to a view of sleep as a complex state which is qualitatively, not just quantitatively, different from wakefulness and which is initiated and maintained by specific mechanisms. Although much progress has been made, the essential nature of sleep remains unresolved, and sleep scientists continue their work.
In their research, sleep scientists need a definition of sleep which need not communicate the essential nature of sleep, but which does provide others with a precise definition of what they mean by sleep as they study it. This definition must be made in terms of observables so that the meanings will not be clouded by the imprecision of words and concepts, and so that several scientists can determine whether their observations are the same. So now the issue becomes, by what observations do we define sleep?
Any scientific working definition of sleep should reflect what we ordinarily mean by "sleep." Therefore, we must ask ourselves what observations we might make of others that would make us decide they were asleep. We do not ordinarily think about this issue, but if we did, we would probably come up with the following four criteria:
Little movement—walking, talking, writing, etc., usually preclude a judgment of sleep.
A stereotypic posture—usually we are lying down when we are asleep, and with rare exception, it is safe to say that people who are, for example, standing on their hands, are not asleep.
A reduced response to stimulation—we do not respond to low intensity sounds, touches, etc., which we would be aware of instantly during wakefulness.
Reversibility—we know that we can readily awake from sleep, which distinguishes it from coma or death.
These criteria constitute a behavioral definition of sleep, which corresponds to the layman's concept of sleep, yet satisfies the scientific requirement of observability.
For several reasons, however, scientists rarely study sleep by observing the behaviors of sleep. One reason is that testing response thresholds and reversibility would interrupt the sleep we want to study. A second reason is that observing and reporting behavior continuously is very cumbersome and time consuming. In practice, the scientist defines sleep by certain physiological measures which are so well correlated with sleep that getting a measure of one provides a pretty good measure of the other. Although these physiological measures derive their value from their correlation with behavioral sleep, they also give us information about different kinds or stages of sleep which are not so apparent in behavioral observations.
We will begin our description of physiological sleep with the sleep of young adult humans. Deviations from these patterns in other species and other age groups will be described in subsequent parts of this syllabus. Traditionally, three primary measures have been used to define physiological sleep and the different physiological sleep stages (figure 1). These are
The electroencephalogram, which is conventionally abbreviated as "EEG" and is popularly known as "brain waves." Hans Berger, a Swiss psychiatrist, discovered the EEG in 1929. He found that small changes in voltage between two small bits of metal, called "electrodes," occurred when they were placed in contact with the scalp. In order to measure these voltage changes, they are powerfully amplified and examined for variations in duration, expressed as Hz (cycles per second, cps) and amplitude, expressed in microvolts (millionths of a volt). The exact physiologic bases of the voltage variations are not entirely known, but it is believed that they emanate largely from changes in voltage of the membranes of nerve cells.
The electrooculogram, which records eye movements, conventionally abbreviated as "EOG." Since the eyeball is like a small battery, with the retina negative relative to the cornea, an electrode placed on the skin near the eye will record a change in voltage as the eye rotates in its socket.
The electromyogram, conventionally abbreviated as "EMG," is a record of the electrical activity which emanates from active muscles. It may also be recorded from electrodes on the skin surface overlying a muscle. In humans, the EMG is typically recorded from under the chin, since muscles in this area show very dramatic changes associated with the sleep stages.
In practice, the EEG, EOG, and EMG are simultaneously recorded on continuously moving chart paper, so that relationships among the three can be seen immediately. The following are examples of the changes typically seen in these three measures during wakefulness and the major sleep stages.
The EEG alternates between two major patterns during wakefulness. One is low voltage (about 10-30 microvolts) fast (16-25 Hz or cps; cycles per second) activity, often called an "activation" or desynchronized pattern. The other is a sinusoidal 8-12 Hz pattern (most often 8 or 12 Hz in college students) of about 20-40 microvolts which is called "alpha" activity. Typically, alpha activity is most abundant when the subject is relaxed and the eyes are closed. The activation pattern is most prominent when subjects are alert with their eyes open and they are scanning the visual environment. REMs may be abundant or scarce, depending on the amount of visual scanning, and the EMG may be high or moderate, depending on the degree of muscle tension.
Polygraphic recording in an awake young adult. EEG=electroencephalogram; EOG=electrooculogram; EMG=electromyogram.
STAGE 1
Alpha activity decreases, activation is scarce, and the EEG consists mostly of low voltage, mixed frequency activity, much of it at 3-7 Hz. REMs are absent, but slow rolling eye movements appear. The EMG is moderate to low.
STAGE 2
Against a continuing background of low voltage, mixed frequency activity, bursts of distinctive 12-14 Hz sinusoidal waves called "sleep spindles" appear in the EEG. Eye movements are rare, and the EMG is low to moderate.
STAGE 3 and 4, delta sleep
High amplitude (>75 mV), slow (0.5-2 Hz) waves called "delta waves" appear in the EEG; EOG and EMG continue as before. In stage 4, there is a quantitative increase in delta waves so that they come to dominate the EEG tracing.
REM
The EEG reverts to a low voltage, mixed frequency pattern similar to that of Stage 1. Bursts of prominent rapid eye movements appear. The background EMG is virtually absent, but many small muscle twitches may occur against this low background.
For the most part, the major differences among Stages 1, 2, 3, and 4 are in their EEG patterns (figure 4). Although there are some exceptions, the general physiology of these stages is fairly similar. In contrast, the physiology of REM sleep is so dramatically different from the other four stages that sleep researchers have distinguished two major kinds of sleep-REM sleep and NREM (non-REM) sleep, which is comprised of Stages 1, 2, 3, and 4.
NREM and REM sleep alternate cyclically through the night. Except in certain pathological conditions, a night of sleep begins with about 80 minutes of NREM sleep, followed by a REM period of about ten minutes. This 90-minute NREM-REM cycle is then repeated about 3-6 times during the night. In the successive cycles of the night, the amounts of Stages 3 and 4 decrease, and the proportion of the cycle occupied by REM sleep tends to increase (figure 5).
It should be clear from this pattern of NREM and REM sleep that sleep is not the simple, uniform suspension of activity which many had assumed it to be for centuries. Rather, sleep shows a complex, highly organized pattern of diverse physiological variables.
This figure is a plot of REM sleep, NREM sleep, and the four stages of NREM sleep over the course of one entire night of sleep. The time spent in NREM sleep is lightly shaded and the time spent in REM sleep is shown in black. NREM sleep always is the first to occur at the beginning of the night. From the beginning of sleep to the end of the first REM period is the first sleep cycle. Thus, the cyclic alternation of NREM and REM sleep constitutes the basic sleep cycle. The average periodicity of this cycle is ninety minutes. Later cycles tend to be a little shorter. As you can see, Stages 3 and 4 dominate the NREM periods in the first part of the night, but are completely absent during the later cycles. Toward the end of the night, very brief periods of wakefulness may interrupt sleep.
How does "depth of sleep" relate to the different stages of sleep? Mostly it doesn't. Depth of sleep, when it is used in a generalized sense to describe the overall character of a portion of sleep, is an archaic concept-a holdover from the old conception of sleep as simply a diminution of waking activity.
If sleep were simply a generalized reduction of waking activity, then it would be sufficient to describe a portion of sleep by the extent of its reduction or "depth." But the facts are that there is no generalized depth of sleep whereby all activities increase or decrease in unison, as described below.
NREM would seem to be "lighter" than REM by the following criteria: subjects awakened from NREM report that they had been in "lighter" sleep than when they are awakened from REM; there is much more galvanic skin activity (sometimes taken as an index of emotional arousal) during NREM than during REM; muscle tone and spinal reflexes can be maintained during NREM, but are severely suppressed during REM; temperature regulation is maintained during NREM, whereas it is severely impaired during REM; in some brain areas, such as the raphe nuclei and locus coeruleus, neuronal activity is maintained during NREM but stops during REM.
REM sleep would seem to be "lighter" in the following respects: in some brain areas, such as the occipital cortex, neural activity is greater during REM than during NREM-sometimes even greater than during wakefulness; whereas brain temperature usually decreases during NREM, in most species it increases during REM; cerebral blood flow increases dramatically when subjects pass from NREM to REM; heart rate, respiration rate, and blood pressure are much more variable during REM than during NREM; the EEG of REM resembles the EEG of wakefulness more than the EEG of NREM does-in fact, in most animals, the EEGs of REM and wakefulness are almost indistinguishable; muscle twitches are more frequent during REM than during NREM; dreaming is reported more frequently on awakenings from REM than on awakenings from NREM; penile erections occur during most REM periods (independent of dream content), but rarely during NREM.
Sleep scientists have not yet figured out why there should be such unique constellations of activity and inactivity in the different kinds of sleep, but it is clear that in neither of the constellations is there a uniform increase or decrease of activity. Therefore, the term "depth of sleep" has little meaning unless it refers to variations in a specifically designated measure. The term has most frequently been used by sleep scientists to designate resistance to being awakened by an external stimulus, such as a sound. For example, in humans, Stages 3 and 4 are "deep" sleep in the specific sense that it takes a louder sound to awaken a subject from these stages than from Stages 1, 2, or REM. However, a louder sound is required to awaken cats and rats from REM than from other stages, so the generality of even this specific depth of sleep measure is limited.
In the face of the numerous ways in which sleep and wakefulness and the different types of sleep differ, one might wonder why the EEG, EOG, and EMG have been widely used as defining criteria of physiological sleep, whereas other variables have generally been described as "correlates" of sleep and sleep stages. In a strict sense, all the measures are correlates. There is nothing holy about the EEG, EOG and EMG. Their choice as defining criteria is based on the following.
Historical precedent. They were among the earliest measures used to discriminate sleep and sleep stages.
Relative ease of measurement. Little specialized equipment is needed, and they can be recorded from surface electrodes. Other measures, such as the firing of individual brain cells, require the surgical implantation of electrodes.
Discriminating power. The EEG, EOG, and EMG in combination do a pretty good job of discriminating sleep from wakefulness and the different sleep stages. Other measures, such as heart rate and respiration rate may vary with wakefulness and the sleep stages, but they do not discriminate the different states as sharply.
Although the EEG, EOG, and EMG measures do a pretty good job of state discrimination, there are occasions when states are not clearly differentiable. State changes do not switch off and on like light switches. Rather, they change more or less gradually, which can make it difficult to draw very sharp dividing lines between states. Even more vexing is the fact that the different processes may change at different rates. For example, during the transition from wakefulness to sleep, there may be several minutes when the EEG looks like that of wakefulness, but awareness of the environment is lost.
More extreme dissociations occur when, for extended periods, some physiological and behavioral criteria indicate one state, while other criteria indicate another state. Examples are sleep talking, sleepwalking, cataplexy (a condition in which subjects suffer from an inhibition of muscle tone, as in REM sleep, but retain a waking EEG and an awareness of the environment), and REM Behavior Disorder (in which muscle inhibition is absent, so that the subject moves vigorously, while the other features of REM sleep are maintained). By convention, some of these states are designated sleep states, as in "sleep" walking, whereas others are designated as a state of wakefulness, as in cataplexy. These verbal conventions obscure the fact that these states reflect sleep and waking processes, which are simultaneously active. It is, therefore, essential that we understand each process and why they show different relationships to each other at different times. One of the major goals of modern sleep research is to understand how sleep and wakefulness interact; another goal is to obtain an in-depth appreciation for what sleep really is!
http://www.medicine.wisc.edu/mainweb/DOMPagesText.php?section=sleepmed&page=whatissleep
All of us know what sleep is. We know how it feels. The meaning of the word is generally not questioned in ordinary conversation, and we do not have to look it up in the dictionary.
But no one knows what sleep really is. Once we get beyond the simple level of recognizing a superficially quiet state that is very different from wakefulness, we find that the major questions about the nature of sleep are largely unanswered. These questions are:
What happens to us while we sleep? At a time before sleep was studied scientifically, laymen and intellectuals alike thought they knew the answer, i.e.., that sleep is a state in which "everything simply slowed down or stopped." That answer is wrong. Although our bodies move less and we make less noise while we sleep, there are many complex changes in our brains and minds during sleep, which cannot be described as a simple reduction of activity. For example, there are nerve cells in our brains, which fire 5 to 10 times more frequently during certain sleep stages than during wakefulness.
Why do we sleep? For millennia, mankind thought it knew the answer. If sleep is a state of inactivity, then it must be for rest. That answer is probably wrong. A few sleep scientists still believe that is true, but most doubt it. More activity during wakefulness does not reliably produce more sleep, and physical rest does not remove the urge to sleep, if anything, it increases the urge. Most answers to the question of why we sleep are theories, not facts.
What are the mechanisms of sleep? So long as we believed that sleep was a state of inactivity that we needed for rest, there was no need to worry about the mechanisms of sleep, about how it got started and was maintained. The apparent answer was that when our bodies and brains got tired, they simply stopped working. That answer is wrong! Although we do not know all the answers about how sleep works, scientists now know that it doesn't occur passively because nothing else is happening. There are mechanisms in the brain that actively induce and maintain sleep. For example, when certain specific parts of the brain are destroyed or inactivated by chemicals, which inhibit nerve action, sleep can be reduced or certain sleep stages can be eliminated entirely.
Thus, our concept of the essential nature of sleep has evolved over the past 35 years from an intuitively appealing, but erroneous, belief that sleep is simply a state of inactivity which occurs passively when organs became fatigued in order they might rest, to a view of sleep as a complex state which is qualitatively, not just quantitatively, different from wakefulness and which is initiated and maintained by specific mechanisms. Although much progress has been made, the essential nature of sleep remains unresolved, and sleep scientists continue their work.
In their research, sleep scientists need a definition of sleep which need not communicate the essential nature of sleep, but which does provide others with a precise definition of what they mean by sleep as they study it. This definition must be made in terms of observables so that the meanings will not be clouded by the imprecision of words and concepts, and so that several scientists can determine whether their observations are the same. So now the issue becomes, by what observations do we define sleep?
Any scientific working definition of sleep should reflect what we ordinarily mean by "sleep." Therefore, we must ask ourselves what observations we might make of others that would make us decide they were asleep. We do not ordinarily think about this issue, but if we did, we would probably come up with the following four criteria:
Little movement—walking, talking, writing, etc., usually preclude a judgment of sleep.
A stereotypic posture—usually we are lying down when we are asleep, and with rare exception, it is safe to say that people who are, for example, standing on their hands, are not asleep.
A reduced response to stimulation—we do not respond to low intensity sounds, touches, etc., which we would be aware of instantly during wakefulness.
Reversibility—we know that we can readily awake from sleep, which distinguishes it from coma or death.
These criteria constitute a behavioral definition of sleep, which corresponds to the layman's concept of sleep, yet satisfies the scientific requirement of observability.
For several reasons, however, scientists rarely study sleep by observing the behaviors of sleep. One reason is that testing response thresholds and reversibility would interrupt the sleep we want to study. A second reason is that observing and reporting behavior continuously is very cumbersome and time consuming. In practice, the scientist defines sleep by certain physiological measures which are so well correlated with sleep that getting a measure of one provides a pretty good measure of the other. Although these physiological measures derive their value from their correlation with behavioral sleep, they also give us information about different kinds or stages of sleep which are not so apparent in behavioral observations.
We will begin our description of physiological sleep with the sleep of young adult humans. Deviations from these patterns in other species and other age groups will be described in subsequent parts of this syllabus. Traditionally, three primary measures have been used to define physiological sleep and the different physiological sleep stages (figure 1). These are
The electroencephalogram, which is conventionally abbreviated as "EEG" and is popularly known as "brain waves." Hans Berger, a Swiss psychiatrist, discovered the EEG in 1929. He found that small changes in voltage between two small bits of metal, called "electrodes," occurred when they were placed in contact with the scalp. In order to measure these voltage changes, they are powerfully amplified and examined for variations in duration, expressed as Hz (cycles per second, cps) and amplitude, expressed in microvolts (millionths of a volt). The exact physiologic bases of the voltage variations are not entirely known, but it is believed that they emanate largely from changes in voltage of the membranes of nerve cells.
The electrooculogram, which records eye movements, conventionally abbreviated as "EOG." Since the eyeball is like a small battery, with the retina negative relative to the cornea, an electrode placed on the skin near the eye will record a change in voltage as the eye rotates in its socket.
The electromyogram, conventionally abbreviated as "EMG," is a record of the electrical activity which emanates from active muscles. It may also be recorded from electrodes on the skin surface overlying a muscle. In humans, the EMG is typically recorded from under the chin, since muscles in this area show very dramatic changes associated with the sleep stages.
In practice, the EEG, EOG, and EMG are simultaneously recorded on continuously moving chart paper, so that relationships among the three can be seen immediately. The following are examples of the changes typically seen in these three measures during wakefulness and the major sleep stages.
The EEG alternates between two major patterns during wakefulness. One is low voltage (about 10-30 microvolts) fast (16-25 Hz or cps; cycles per second) activity, often called an "activation" or desynchronized pattern. The other is a sinusoidal 8-12 Hz pattern (most often 8 or 12 Hz in college students) of about 20-40 microvolts which is called "alpha" activity. Typically, alpha activity is most abundant when the subject is relaxed and the eyes are closed. The activation pattern is most prominent when subjects are alert with their eyes open and they are scanning the visual environment. REMs may be abundant or scarce, depending on the amount of visual scanning, and the EMG may be high or moderate, depending on the degree of muscle tension.
Polygraphic recording in an awake young adult. EEG=electroencephalogram; EOG=electrooculogram; EMG=electromyogram.
STAGE 1
Alpha activity decreases, activation is scarce, and the EEG consists mostly of low voltage, mixed frequency activity, much of it at 3-7 Hz. REMs are absent, but slow rolling eye movements appear. The EMG is moderate to low.
STAGE 2
Against a continuing background of low voltage, mixed frequency activity, bursts of distinctive 12-14 Hz sinusoidal waves called "sleep spindles" appear in the EEG. Eye movements are rare, and the EMG is low to moderate.
STAGE 3 and 4, delta sleep
High amplitude (>75 mV), slow (0.5-2 Hz) waves called "delta waves" appear in the EEG; EOG and EMG continue as before. In stage 4, there is a quantitative increase in delta waves so that they come to dominate the EEG tracing.
REM
The EEG reverts to a low voltage, mixed frequency pattern similar to that of Stage 1. Bursts of prominent rapid eye movements appear. The background EMG is virtually absent, but many small muscle twitches may occur against this low background.
For the most part, the major differences among Stages 1, 2, 3, and 4 are in their EEG patterns (figure 4). Although there are some exceptions, the general physiology of these stages is fairly similar. In contrast, the physiology of REM sleep is so dramatically different from the other four stages that sleep researchers have distinguished two major kinds of sleep-REM sleep and NREM (non-REM) sleep, which is comprised of Stages 1, 2, 3, and 4.
NREM and REM sleep alternate cyclically through the night. Except in certain pathological conditions, a night of sleep begins with about 80 minutes of NREM sleep, followed by a REM period of about ten minutes. This 90-minute NREM-REM cycle is then repeated about 3-6 times during the night. In the successive cycles of the night, the amounts of Stages 3 and 4 decrease, and the proportion of the cycle occupied by REM sleep tends to increase (figure 5).
It should be clear from this pattern of NREM and REM sleep that sleep is not the simple, uniform suspension of activity which many had assumed it to be for centuries. Rather, sleep shows a complex, highly organized pattern of diverse physiological variables.
This figure is a plot of REM sleep, NREM sleep, and the four stages of NREM sleep over the course of one entire night of sleep. The time spent in NREM sleep is lightly shaded and the time spent in REM sleep is shown in black. NREM sleep always is the first to occur at the beginning of the night. From the beginning of sleep to the end of the first REM period is the first sleep cycle. Thus, the cyclic alternation of NREM and REM sleep constitutes the basic sleep cycle. The average periodicity of this cycle is ninety minutes. Later cycles tend to be a little shorter. As you can see, Stages 3 and 4 dominate the NREM periods in the first part of the night, but are completely absent during the later cycles. Toward the end of the night, very brief periods of wakefulness may interrupt sleep.
How does "depth of sleep" relate to the different stages of sleep? Mostly it doesn't. Depth of sleep, when it is used in a generalized sense to describe the overall character of a portion of sleep, is an archaic concept-a holdover from the old conception of sleep as simply a diminution of waking activity.
If sleep were simply a generalized reduction of waking activity, then it would be sufficient to describe a portion of sleep by the extent of its reduction or "depth." But the facts are that there is no generalized depth of sleep whereby all activities increase or decrease in unison, as described below.
NREM would seem to be "lighter" than REM by the following criteria: subjects awakened from NREM report that they had been in "lighter" sleep than when they are awakened from REM; there is much more galvanic skin activity (sometimes taken as an index of emotional arousal) during NREM than during REM; muscle tone and spinal reflexes can be maintained during NREM, but are severely suppressed during REM; temperature regulation is maintained during NREM, whereas it is severely impaired during REM; in some brain areas, such as the raphe nuclei and locus coeruleus, neuronal activity is maintained during NREM but stops during REM.
REM sleep would seem to be "lighter" in the following respects: in some brain areas, such as the occipital cortex, neural activity is greater during REM than during NREM-sometimes even greater than during wakefulness; whereas brain temperature usually decreases during NREM, in most species it increases during REM; cerebral blood flow increases dramatically when subjects pass from NREM to REM; heart rate, respiration rate, and blood pressure are much more variable during REM than during NREM; the EEG of REM resembles the EEG of wakefulness more than the EEG of NREM does-in fact, in most animals, the EEGs of REM and wakefulness are almost indistinguishable; muscle twitches are more frequent during REM than during NREM; dreaming is reported more frequently on awakenings from REM than on awakenings from NREM; penile erections occur during most REM periods (independent of dream content), but rarely during NREM.
Sleep scientists have not yet figured out why there should be such unique constellations of activity and inactivity in the different kinds of sleep, but it is clear that in neither of the constellations is there a uniform increase or decrease of activity. Therefore, the term "depth of sleep" has little meaning unless it refers to variations in a specifically designated measure. The term has most frequently been used by sleep scientists to designate resistance to being awakened by an external stimulus, such as a sound. For example, in humans, Stages 3 and 4 are "deep" sleep in the specific sense that it takes a louder sound to awaken a subject from these stages than from Stages 1, 2, or REM. However, a louder sound is required to awaken cats and rats from REM than from other stages, so the generality of even this specific depth of sleep measure is limited.
In the face of the numerous ways in which sleep and wakefulness and the different types of sleep differ, one might wonder why the EEG, EOG, and EMG have been widely used as defining criteria of physiological sleep, whereas other variables have generally been described as "correlates" of sleep and sleep stages. In a strict sense, all the measures are correlates. There is nothing holy about the EEG, EOG and EMG. Their choice as defining criteria is based on the following.
Historical precedent. They were among the earliest measures used to discriminate sleep and sleep stages.
Relative ease of measurement. Little specialized equipment is needed, and they can be recorded from surface electrodes. Other measures, such as the firing of individual brain cells, require the surgical implantation of electrodes.
Discriminating power. The EEG, EOG, and EMG in combination do a pretty good job of discriminating sleep from wakefulness and the different sleep stages. Other measures, such as heart rate and respiration rate may vary with wakefulness and the sleep stages, but they do not discriminate the different states as sharply.
Although the EEG, EOG, and EMG measures do a pretty good job of state discrimination, there are occasions when states are not clearly differentiable. State changes do not switch off and on like light switches. Rather, they change more or less gradually, which can make it difficult to draw very sharp dividing lines between states. Even more vexing is the fact that the different processes may change at different rates. For example, during the transition from wakefulness to sleep, there may be several minutes when the EEG looks like that of wakefulness, but awareness of the environment is lost.
More extreme dissociations occur when, for extended periods, some physiological and behavioral criteria indicate one state, while other criteria indicate another state. Examples are sleep talking, sleepwalking, cataplexy (a condition in which subjects suffer from an inhibition of muscle tone, as in REM sleep, but retain a waking EEG and an awareness of the environment), and REM Behavior Disorder (in which muscle inhibition is absent, so that the subject moves vigorously, while the other features of REM sleep are maintained). By convention, some of these states are designated sleep states, as in "sleep" walking, whereas others are designated as a state of wakefulness, as in cataplexy. These verbal conventions obscure the fact that these states reflect sleep and waking processes, which are simultaneously active. It is, therefore, essential that we understand each process and why they show different relationships to each other at different times. One of the major goals of modern sleep research is to understand how sleep and wakefulness interact; another goal is to obtain an in-depth appreciation for what sleep really is!
http://www.medicine.wisc.edu/mainweb/DOMPagesText.php?section=sleepmed&page=whatissleep