The Science of Sleep: What It Is, How It Works, and Why It Matters
176
The Science of Sleep: What It Is, How It Works, and Why It Matters
176Hardcover
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Product Details
| ISBN-13: | 9780226387161 |
|---|---|
| Publisher: | University of Chicago Press |
| Publication date: | 10/06/2017 |
| Pages: | 176 |
| Product dimensions: | 8.60(w) x 9.60(h) x 0.80(d) |
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CHAPTER 1
HUMAN SLEEP
We can measure our lives on many different levels. A life, for instance, could be considered the period in which a person's heart beats a certain number of times (perhaps 3 billion), or enjoys Saturday nights (about 3,600), or experiences periods of peacefulness (all too few), or feels the joy of love (hopefully at least once). In the same way, sleep, which occupies perhaps one-third of our existence, can be viewed on a number of different levels. It is a process that can be measured physiologically while at the same time understood as a psychological experience, and even a social behavior. In this book we will explore some of these aspects of sleep. Human sleep is a reversible period of decreased consciousness and responsiveness, comprised of two distinct states known as rapid eye movement (REM) and non-rapid eye movement (NREM) sleep. Although the modern era of sleep research began in the 1950s with the description of REM sleep, its roots go back to the 1920s with the discovery of the human electroencephalogram, or EEG. Our growing understanding of sleep has been influenced by some remarkable individuals who (sometimes while looking for something else) made crucial observations about sleep, by developments in psychology and technology, and even by world events.
WHAT IS SLEEP?
The many qualities of sleep
Sleep is a period of recurring behavioral quiescence, which has several qualities, including decreased awareness of and responsiveness to the environment, diminished consciousness, and the rhythmic appearance of certain physiologic patterns (stages). It tends to occur at particular parts of the 24-hour day–night cycle, and at customary locations, both depending on the particular species and environment. It is reversible, distinguishing it from coma or ongoing anesthesia. It is also self-regulating: if one is deprived of sleep, there will be a drive to have increased "recovery sleep" to make up for the loss. It is necessary for life, and is present in all mammals. These qualities will be discussed later in the chapter.
DIMINISHED CONSCIOUSNESS
The nature of diminished consciousness in sleep is by far the hardest to express, since consciousness itself is so poorly understood. When we speak of consciousness, we refer to our experience of self and the world. Perhaps even more fundamentally, consciousness can be defined as "our mode of access." We can additionally speak of "acts" of consciousness, such as perceiving, willing, imagining, and the "contents" of these acts, such as perceiving a sunset, willing a meeting, or imagining a good outcome.
Other efforts to characterize consciousness are helpful. For example, consciousness has been described as a paradoxical state in which a person is simultaneously a subject who can experience things and an object perceived by oneself. Generally most of us recognize this when we speak of ourselves in both ways simultaneously, for instance when we say, "Sometimes I have to remind myself that ..." or "I owe it to myself to ...". In such phrases we recognize that we are both a being having experiences and at the same time we can picture ourselves as objects.
Additional qualities of consciousness include consciousness as subjective and private (not available to another person); unitary (experienced by a single person); and characterized by a subjective "feel" of each experience, a "what it is like" aspect. The American philosopher Thomas Nagel (1937–) illustrated the "feel" which is essential to consciousness in the following manner: although we may study and understand the neurophysiology of a bat, we can never know how the world is experienced by a bat. Some authors argue that a subjective experience such as consciousness cannot profitably be studied by traditional scientific techniques; others believe that the fact that a phenomenon is experienced subjectively does not mean that it cannot be explored objectively.
New developments in technology, including the use of brain-imaging studies, are beginning to advance our understanding of the physiology behind consciousness, and researchers have developed a number of models of how it may occur. In addition to looking at normal sleep, neuroimaging studies of people who have been given hallucinogens such as psilocybin have contributed new theories of consciousness based on the notion of entropy — the degree of order and disorder in neural connections.
During sleep one becomes less aware of the surroundings; this is one of the qualities that distinguishes sleep from quiet wakefulness. An extreme case would be someone who falls asleep at the wheel, consequently running a red light. On the other hand, this process is not absolute; it is clear that we can process, and act on, sensory information during sleep. A new parent, for instance, can sleep through the noise of a truck driving by the house, but awaken quickly at the sound of the newborn baby crying. Similarly, in a laboratory situation, the volume of a sound required to wake a person up (the "auditory arousal threshold") is much higher for a meaningless sound such as an electronic tone compared to a meaningful stimulus such as a phone ringing or hearing one's name called. A sleeper, then, is able to process information about an incoming sound, and determine whether it is important or not. Again, the arousal response is dependent on a variety of factors including the sleep stage, the duration of wakefulness before sleep, and how far into sleep one is when the sound occurs. Some individuals are more likely to be awakened by low volume noises, and hence are considered to be "light sleepers." Interestingly, people with insomnia often have a normal auditory arousal threshold, suggesting that insomnia is different from just light sleep. Some sleeping pills increase the auditory arousal threshold, raising the possibility that the medicated sleeper will not be aroused by, for instance, a smoke alarm.
Time-lapse videos show that two sleepers adjust their positions in response to the other's movements (whether the other sleeper is a pet or another human). In one film, for instance, a man sleeping with a cat on the bed may turn on his side, with the cat then responding by settling comfortably in the warm nook behind his knees. In co-sleeping humans, an elbow in the ribs may result in an adjustment of the second person's position. Thus, although sensory input is diminished during sleep, this is not absolute, and indeed during sleep we are able to take in and act on information to some degree.
SLEEP STUDIES
This composite picture shows a subject undergoing a sleep recording across the night. The scientific study of sleep continues to reveal more and more insights into what happens while we are asleep, and how it influences our waking lives.
HOW IS THE ELECTRICAL ACTIVITY OF THE BRAIN MEASURED?
Introduction to the Electroencephalogram (EEG)
Sleep is comprised of rhythmically recurring sleep stages, which are defined by looking at brain waves, or an electroencephalogram (EEG), in conjunction with the electrooculogram (EOG), which records eye movements and the electromyogram (EMG), a measure of muscle activity.
The discovery of brain waves
Before the sleep stages are described, it is useful to look at how the brain waves were discovered, and what they are like. It was discovered in the 1820s that electrical current could cause the needle of a magnetic compass to fluctuate, and that this effect could be multiplied by the use of coils of wire. An instrument based on this observation was known as a galvanometer, named after Luigi Galvani (1737–98), an Italian physician and biologist who in 1791 observed that electrical currents could cause the limbs of a dead frog to twitch. This was one of the first observations that electrical currents might be involved in biological processes. Some years later, Richard Caton (1842–1926), a Scottish physiologist, used a galvanometer to detect electrical current from the brains of dogs and apes. In 1875 he reported that the current differed at various times, increasing in strength during sleep and preceding death, then disappearing upon death.
A half century later, Hans Berger (1873–1941), a German psychiatrist, made the next great advance. Berger had been a mathematics student who dropped out and enlisted in the cavalry. One day, he was thrown from his horse, landing in the path of an oncoming cannon carriage, which barely managed to stop at the last moment. At the same time that Berger had this life-threatening experience, his sister living some distance away is said to have had a sudden sensation that he was in danger, and urged her father to contact him. Berger was so struck by this apparent "psychic energy" alarming his sister, that he went on to devote his life to exploring the brain and how objective measures of its activity might relate to subjective psychic processes. In 1929, he described recordings of the electrical waves in humans that had been reported by Caton in animals, and developed a way to record them on moving strips of paper. He coined the term "electroencephalogram" to describe his new discovery. He showed that the waves differed in waking, in sleep, and in anesthesia, and that sharp spiking patterns appeared during epileptic seizures. He described the alpha rhythm in a subject resting with closed eyes, and its disappearance and replacement by faster beta waves when the eyes were opened. These waveforms over the years have been divided into several wavebands according to their frequency (number of waves per second). Other information used in describing them involves their shape, amplitude (a measure of their energy), and location on the head (see also "Features of an oscillating system," shown here). Subsequent to Berger's work focusing on alpha and beta, a series of EEG bands have been further defined and are generally recognized to this day.
MAJOR EEG BANDS
Delta waves (0.5–4 Hz)
These slow waves have a frequency range of 0.5–4 cycles per second (known as cps or Hz). Our interest in them from the point of view of sleep is that they are characteristic of stage N3 — also known as slow-wave sleep (SWS), or stages 3 and 4. In clinical EEG work in waking patients, in distinction from sleep studies, they can be associated with the presence of lesions in local areas of the brain, or can appear in a widespread manner in diffuse disorders.
Theta waves (4.5–8 Hz)
These waves appear during lighter sleep. In waking clinical EEG work they are considered a sign of sleepiness, increasing with the duration of wakefulness. They can also appear during hyperventilation. In sleep studies, they are an important rhythm in stage N2 ( stage 2) sleep.
Alpha waves (8.5–12 Hz)
Also known as "Berger waves," these are best seen occipitally (in the back of the head), and are manifested in a relaxed but awake person with eyes closed. As mentioned before, they greatly decrease when the eyes are opened. When eyes are closed in an awake person, they decrease as one becomes sleepy.
Beta waves (12.5–30 Hz)
Beta waves, which are more evident anteriorly (toward the front of the head), are often divided into irregular, disorganized, low amplitude and organized waveforms. The former is primarily seen in an awake person who is actively thinking, concentrating, or feeling anxiety. The latter is seen in some illnesses or as a result of some sedatives including barbiturates or Valium-like drugs (benzodiazepines). All EEG bands (including beta) can be viewed visually or measured electronically. The amount of electronically measured beta activity is sometimes considered a measure of arousal of the cerebral cortex.
THE SLEEP STAGES
Initial discoveries
The next step in the evolution of understanding sleep was the recognition that EEG brainwaves, when combined with other kinds of physiological information, could be used to identify rhythmically recurring, discrete sleep stages. The first description of these sleep stages was made by the American scientist Alfred Loomis (1887–1975). As a young man in the army he developed the Aberdeen Chronograph, a system for measuring projectile velocity by firing a bullet through revolving paper-covered aluminum disks. He went on to a successful career in investment banking. Becoming restless once again, and still remembering his success with the chronograph, he turned his attention to developing radar for military purposes and for ground control during the landing approach of aircraft. He was fascinated with the measuring of waveforms and, among the many projects at his laboratory at Tuxedo Park, New York, was the study of sleep. Using a large 8 foot (2.4 m) diameter recording drum, he described in 1937 a series of five discrete recurring sleep stages during the night, which he rather unpoetically designated as stages A–E. In terms of later development, stages A and B correspond roughly to what was later called stage 1, C corresponded to stage 2, and D and E resembled slow-wave sleep. All together they correspond to what we now call non-rapid eye movement (NREM) sleep.
The discovery of rapid eye movement (REM) sleep
The next big development, which in effect ushered in the modern age of sleep research, occurred in the early 1950s. Nathaniel Kleitman (1895–1999), a physiologist at the University of Chicago, had been interested in eye movements and blinking as a marker of sleep onset and depth of sleep, as well as possible rhythmic behaviors, in infants. He enlisted the aid of a graduate student, Eugene Aserinsky (1921–98). Following observations in infants, they adapted the technique of the electrooculogram (EOG) for continuous use in sleep of children and adults. In the process of doing so, they observed the periodic appearance of vigorous and jerky ocular activity. This new stage, known as rapid eye movement (REM) sleep, was characterized not only by the eponymous eye movements, but also by relaxation of the major weight-bearing muscles, irregularity of respiratory and heart rate, and loss of temperature control. It also has psychological counterparts, and most dreaming, in the conventional sense of the word, occurs in REM. Indeed, REM sleep is as different from the rest of sleep (dubbed by Kleitman as non-REM or NREM sleep) as NREM is from waking. This has led some authors to describe humans as having three distinct states of consciousness: waking, REM, and NREM sleep.
The sleep stages do not appear randomly, but instead are manifest in a rhythmic, repetitive pattern throughout the night. There are a number of influences on the appearance and duration of the individual stages, including a basic approximately 90–100-minute innate rhythmic cycle of NREM and REM sleep (an example of "ultradian rhythms," see here), the time of the 24-hour day at which sleep occurs, and the duration of wakefulness before sleep. In the next section we will describe the sleep stages in much more detail, but perhaps the important message at this point is that sleep is not a unitary process, but is comprised of two very distinct states as well as several distinct stages in NREM sleep.
HOW IS SLEEP ORGANIZED INTO STAGES?
The evolution of sleep staging
The first major revision of sleep staging after Loomis was developed in 1968 by the American scientists Alan Rechtschaffen (1928– ) and Anthony Kales (1934– ); their Manual of Standardized Terminology, Techniques and Scoring System for Sleep Stages of Human Subjects was the standard classification until 2007, when the American Academy of Sleep Medicine (AASM) made revisions which are in effect today and used internationally. The Rechtschaffen–Kales criteria were largely oriented to the use of polygraphs recording with ink pens on paper. Since a page of the typical EEG fan-fold paper was 30 cm wide, ultimately the most common recording speed was with the paper moving 10 mm/sec, so that a 30-second view of sleep was seen on each page. Thus sleep was scored in 30-second "epochs," that is, for each 30-second page, a decision was made of what predominant sleep stage was present. Since a box of paper was typically 1,000 pages long, one night's sleep could be recorded on one box. The signal moving across the paper could be magnified to the degree desired. EEGs are very weak signals measured in millionths of a volt (microvolts or uV); for comparison, electrocardiogram signals from the heart as detected on the skin are roughly one thousand times stronger and are measured in thousandths of a volt (millivolts). Incidentally, the 30-second sleep stage scoring epochs are often used to this day even with modern electronic equipment: it is a good example of how past solutions are carried forward even when new technologies are developed. It's the same reason that now even in the digital age, most popular songs still last about 3 minutes (the duration of recording time that was available on old 45 RPM vinyl discs).
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