I INTRODUCTION Sleep, normal, regular state of rest of an organism. In contrast to the waking state, sleep is characterized by relative quiescence of physiological functions (blood pressure, breathing, heartbeat) and a relatively low response to external stimuli.
II TYPICAL
SLEEP CYCLE
Sleep has long been treated as a behavioral state.
Nonetheless, some characteristics of brain physiology such as brain-wave
patterns—as recorded by electroencephalography, or EEG—are now accepted as
part of its definition because of their unvarying association with sleep
behavior.
A Stages
of Sleep
The brain waves of a person go through certain constant
changes, classified as stages 1 to 4, in the course of the sleep cycle. The EEG
of a person in the waking state is characterized by alpha waves (8 to 12
cycles/sec) and low-voltage activity of mixed frequency, whereas sleep onset
involves a disappearance of this alpha activity. Stage 1, considered the
lightest stage of sleep, is characterized by low-voltage, desynchronized
activity and sometimes by low-voltage, regular activity at 4 to 6 cycles/sec as
well. After a few seconds or minutes, this gives way to stage 2, a pattern
showing frequent spindle-shaped tracings on the EEG, called sleep spindles, at
13 to 15 cycles/sec, and certain high-voltage spikes known as K-complexes. Soon
thereafter, stage 3 begins with the appearance of delta waves (high-voltage
activity at 0.5 to 2.5 cycles/sec). Eventually, in stage 4, these delta waves
occupy the major part of the record.
B Dreaming
Sleep
The categorization of sleep records into these four
stages is a somewhat arbitrary division of a continuous process. More important,
sleep exhibits four or five periods of so-called emergence from stages, 2, 3,
and 4 to a stage similar to stage 1. Persons awakened during these periods of
emergence frequently—60 to 90 percent of the time—report that they have been
dreaming. Such periods are characterized not only by stage-1 EEG patterns and by
rapid conjugate (coupled) eye movements but also by many other distinguishing
factors. Among these are a great irregularity in pulse rate, respiratory rate,
and blood pressure; the presence of full or partial penile erections in the
male; and generalized low muscular tone interrupted by movements in small muscle
groups.
Periods of emergence thus differ markedly from typical stage-1 sleep as well as from the other three stages. Because of these distinguishing characteristics and because of their specific neurophysiological and chemical character (see below), these periods are now almost universally seen as constituting a separate state of sleep. Similar sleep periods are found in nearly all mammals and birds studied. These periods are referred to as D- (desynchronized or dreaming) sleep; the remainder of sleep is called S- (synchronized) sleep. These two states are also known, respectively, as REM (rapid-eye-movement) sleep and NREM (non-rapid-eye-movement) sleep; as paradoxical sleep and orthodox sleep; or as active sleep and quiet sleep.
C Anomalies
The individual EEG changes characteristic of sleep may
sometimes be deceptive. The deep, slow waves usually associated with sleep, for
instance, can be found in the waking state under certain pharmacological
conditions and are also seen during certain phases of anesthesia or coma. Thus,
when an EEG tracing is used to make a diagnosis of sleep, the regular cyclic
pattern described above and the regular alternation of the two states are the
most important phenomena, rather than any single characteristic wave form.
D Time
Allotments
Several characteristics of a typical night's sleep are
found regularly and universally. First, the four or five periods of D-sleep that
occur during the night take up a total time of about 90 minutes, a little more
than 20 percent of total sleep time. Second, the first D-sleep period occurs
about 70 to 120 minutes after the onset of sleep. This interval may be longer in
some normal subjects, but it is significantly shorter only in a few abnormal
clinical and experimental conditions such as narcolepsy; and the pattern occurs
whether or not the person recalls any dreams.
Some time variations occur with age, however. The young
always have more sleep time, and considerably more D-sleep time, than do adults,
as in any mammalian species. The newborn child sleeps 16 to 18 hours, at least
half of which is D-sleep. The young adult human spends 16 to 17 hours awake and
7 to 8 hours asleep, of which perhaps 6 hours are spent in S-sleep and 1.5 hours
in D-sleep. Both S- and D-sleep, on the average, decrease slightly with
increasing age. The same relationship appears to hold for other mammalian
species.
E Sleep
in Other Species
As for other animals, most vertebrates may be said to
display at least a primitive form of sleep, according to the above behavioral
definition. Fish and amphibians have periods of quiescence accompanied by
decreased response to environmental stimuli. Concomitant EEG or other
recordings, however, have not demonstrated clear-cut sleep-versus-waking
differences. Reptiles demonstrate sleep behavior, and recordings show results
somewhat similar to mammalian S-sleep; in a few instances, brief episodes of a
state very much resembling D-sleep have been recorded as well. Birds have
definite periods of both S- and D-sleep, although the D-periods are generally
very short and account for a small percentage of total sleep time. All mammals
have clear S- and D-sleep, with the possible exception of a single very
primitive mammal, the spiny anteater.
III PHYSIOLOGY
AND CHEMISTRY OF SLEEP
A tremendous amount of knowledge has been accumulated
about the central and peripheral mechanisms controlling and involving sleep.
Basically, certain areas in the brain stem—the most primitive part of the
brain and the part that controls such basic functions as breathing and heart
rate—are involved in the control of the two sleep states. Considerable
controversy still exists as to exactly which brain-stem regions are involved and
how they interact, but it is known that several brain chemicals called biogenic
amines—dopamine, norepinephrine, and serotonin—act as neurotransmitters and
neuromodulators in regulating discharge of brain cells. The evidence is clearest
for the involvement of serotonin. Serotonin is necessary for normal sleep to
occur, although it is only one of many elements and is not sufficient in itself.
The roles that norepinephrine and dopamine have in sleep are less certain.
Recent research demonstrates that the human nervous system controls the body's functions differently during the sleep states than during waking. The details are complex, but breathing mechanisms, temperature mechanisms, and musculature all function differently during sleep. Especially dramatic are the changes during D-sleep, in which core-body temperature is hardly controlled at all, so that mammals, including humans, become poikilothermic (cold-blooded). Differences in control mechanisms are becoming important in helping to characterize and understand a whole series of sleep-related diseases; for instance, in sleep apnea, breathing repeatedly stops or becomes very shallow during sleep.
A Functions of Sleep and Sleep Requirements Probably the most important and difficult question is that of the functions of sleep. This question has not been completely answered, and differences of opinion exist. Some scientists believe that sleep has no biological function and is simply a sort of habit. The predominance of evidence, however, suggests a biological function for sleep—in fact, most probably two functions, related to the two states of sleep. S-sleep tends to increase after exercise, after starvation, and at other times of increased metabolic need. Thus, S-sleep probably plays a role in the restoration of the body and brain, perhaps facilitating the synthesis of large molecules such as proteins and ribonucleic acids (see Nucleic Acids). D-sleep may play a more complex role in providing restoration for brain processes—especially some higher-level brain processes involved in focusing attention, waking ego mechanisms, performing subtle cognitive and social tasks, and so on.
The numerous investigations leading to these conclusions include studies of total sleep deprivation and of differential deprivation of different sorts of sleep, as well as studies of persons who always sleep 9 or more hours (long sleepers) and those who always sleep less than 6 hours (short sleepers). As the latter point indicates, a tremendous variation occurs in sleep requirements. Some persons function well on five hours of sleep a night, whereas others require ten hours; yet they are all physically and mentally normal. A person functioning with no sleep or almost none is occasionally heard of, but such reports have not been substantiated; apparently some sleep, at least four or five hours, is needed by everyone.
B Sleep Disorders A new field of clinical medicine is developing, related to psychiatry and neurology but not identical to either one. Called sleep medicine, it deals with sleep disorders, of which many kinds can be identified. Sleep problems are usually divided into three kinds: the insomnias, a group of problems producing difficulty in falling asleep or difficulty in staying asleep (see Insomnia); hypersomnolence, characterized by too much sleep, or sleepiness when a person does not want to sleep (see Narcolepsy); and episodic nocturnal events, consisting of disorders such as night terrors, nightmares, and sleepwalking (see Somnambulism).
Insomnia and hypersomnolence are only symptoms and may have many different causes. For example, insomnia can be caused by such conditions as painful arthritis; by endocrine disturbances; by the use of certain chemical substances or by the withdrawal from others (including alcohol); by psychological problems, such as anxiety and depression; and by disturbances in biorhythm such as jet lag (see Biological Clocks). In terms of treatment, therefore, insomnia is not an illness that can be cured by a sleeping pill. Rather, the physician must determine and treat the insomnia's underlying cause.
Contributed By:
Ernest Louis Hartmann