Sleep and dreaming - AccessScience from McGraw-Hill Education http://accessscience.com/content/sleep-and-dreaming/628000

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Article by: LaBerge, Stephen The Lucidity Institute, Palo Alto, California. Publication year: 2012 DOI: http://dx.doi.org/10.1036/1097-8542.628000 (http://dx.doi.org/10.1036/1097-8542.628000)

Content

Nature of sleep Evolution and function of sleep Bibliography Sleep cycle Dreams Additional Readings Measuring sleepiness

A state of rest in which consciousness and activity are diminished, and in which an involuntary series of visual, auditory, or kinesthetic images, emotions, and thoughts occur in the mind, which take the form of a sequence of events or of a story, having a feeling of reality but totally lacking a feeling of free will. Sleep is generally defined as an easily reversible, temporary, periodic state of suspended behavioral activity, unresponsiveness, and perceptual disengagement from the environment. Compared with other states of temporary unresponsiveness such as syncope or coma, sleep is easily reversible with strong or meaningful sensory stimuli (for example, the roar of a nearby tiger, or a voice speaking the sleeper's name). Sleep should not be considered a state of general unconsciousness. The sleeper is normally unconscious (but not always) of the nature of events in the surrounding environment; this is the meaning of perceptual disengagement: a lack of conscious perception and meaningful responsiveness to environmental stimuli. However, the sleeper's attention may be fully engaged in experiencing a dream. Furthermore, if reportability is accepted as a sufficient condition for conscious mental processes, any dream that can be recalled must be considered conscious. Dreaming, then, can be simply defined as the world-modeling constructive process through which people have experiences during sleep, and a dream is just whatever the dreamer experienced while sleeping.

Nature of sleep

In general, biological organisms do not remain long in states of either rest or activity. For example, if a cat's blood sugar level drops below a certain point (the cat's reference level for hunger-related action), the cat is motivated by hunger to venture from its den in search of a meal. It will continue to hunt until it finds food (or starves). After satisfying the urge to eat, the cat is no longer motivated to expend energy tracking down uncooperative prey; now its biochemical state motivates a return to its den, to digest in peace, conserve energy, and generally engage in restful, regenerative activities, including sleep. This example tracks a cat through one cycle of its basic rest-activity cycle (BRAC). Such cyclic processes are ubiquitous among living systems.

Sleep and wakefulness are complementary phases of the most salient aspects of the brain's endogenous circadian rhythm, or biological clock. Temporal isolation studies have determined the biological clock in humans to be slightly longer than 24 h. Several features of sleep are regulated by the circadian system, including sleep onset and offset, depth of sleep, and rapid eye movement (REM) sleep intensity and propensity. In the presence of adequate temporal cues (for example, sunlight, noise, social interactions, and alarm clocks), the internal clock keeps good time, regulating a host of physiological and behavioral processes. See also: Biological clocks (/content/biological-clocks/082550); Noradrenergic system (/content

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Sleep is not a uniform state of passive withdrawal from the world, as scientists thought until the twentieth century. In fact, a version of the BRAC continues during sleep, showing a periodicity of approximately 90 minutes. There are two distinct kinds of sleep: a quiet phase (also known as quiet sleep or QS, slow-wave sleep) and an active phase (also known as active sleep or AS, REM sleep, paradoxical sleep), which are distinguished by many differences in biochemistry, physiology, psychology, and behavior. Recordings of electrical activity changes of the brain (electroencephalogram or EEG), eye movements (electrooculogram or EOG), and chin muscle tone (electromyogram or EMG) are used to define the various stages and substages of sleep (Fig. 1). See also: Electroencephalography (/content/electroencephalography/221300)

Fig. 1 Standard electrode placements for sleep research. Electroencephalogram (EEG) waves are recorded from left (C3) or right (C4) central locations, which give the best discrimination of sleep stages. Electrooculogram (EOG) electrodes are typically sited to allow both vertical and horizontal eye movements to be recorded. The standard chin (submental) electromyogram (EMG) site is chosen to show reliable suppression of muscle tone during onset of REM (rapid eye movement) sleep.

Sleep cycle

If sleepy enough, most people can fall asleep under almost any condition, even while sitting up or being subjected to loud noise, bright lights, and painful electric shocks. However, humans usually lie down and decrease environmental stimulation when preparing to go to sleep. After lying in bed for a few minutes in a quiet, dark room, drowsiness usually sets in. The subjective sensation of drowsiness can be objectively indexed by a corresponding change in brain waves (EEG activity): formerly continuous alpha rhythms (Fig. 2a) gradually break up into progressively shorter trains of regular alpha waves and are replaced by low-voltage mixed-frequency EEG activity. When less than half of an epoch [usually the staging epoch is the 20–30 seconds it takes to fill one page of polygraph (sleep recording) paper] is occupied by continuous alpha rhythm, sleep onset is considered to have occurred and stage 1 sleep is scored (Fig. 2b). At this stage, the EOG usually reveals slowly drifting eye movements (SEMs) and muscle tone might or might not decrease. Awakenings at this point frequently yield reports of hypnagogic (leading into sleep) imagery, which can often be extremely vivid and bizarre.

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Fig. 2 Human EEG associated with different stages of sleep and wakefulness. (a) Relaxed wakefulness (eyes shut) shows rhythmic 8–12-Hz alpha waves. (b) Stage 1 non-REM sleep shows mixed frequencies, especially 3–7-Hz theta waves. (c) Stage 2 non-REM sleep shows 12–14-Hz sleep spindles and K-complexes. (d) Delta sleep shows large-amplitude (>75 μV) 0.5–2-Hz delta waves. (e) REM sleep shows low-amplitude, mixed frequencies with sawtooth waves.

Stage 1 is a very light stage of sleep described by most subjects as “drowsing” or “drifting off to sleep.” Normally, it lasts only a few minutes before further EEG changes occur, defining another sleep stage. It is at this point that startlelike muscle jerks known as hypnic myoclonias or hypnic jerks occasionally briefly interrupt sleep. As the subject descends deeper into sleep, the EEG of stage 2 sleep is marked by the appearance of relatively high-amplitude slow waves called K-complexes as well as 12–14-Hz rhythms called sleep spindles (Fig. 2c). The EOG would generally indicate little eye movement activity, and the EMG would show somewhat decreased muscle tone. Reports of mental activity from this stage of sleep are likely to be less bizarre and more realistic than those from stage 1. However, light sleepers sometimes report lengthy and vivid dreams upon awakening from stage 2 sleep, especially late in the sleep cycle.

After several minutes in stage 2, high-amplitude slow waves (delta waves) gradually begin to appear in the EEG. When at least 20% of an epoch is occupied by these (1–2-Hz) delta waves, stage 3 is defined. Usually this slow-wave activity continues to increase until it completely dominates the appearance of the EEG. When the proportion of delta EEG activity exceeds 50% of an epoch, the criterion for the deepest stage of sleep, stage 4, is met. During stages 3 and 4, often collectively referred to as delta sleep (Fig. 2d), the EOG shows few genuine eye movements but is obscured by the high-amplitude delta waves. Muscle tone is normally low, although it can be remarkably high, as when sleepwalking or sleep- talking occurs. Recall of mental activity on arousal from delta sleep is generally very poor and fragmentary and is more thoughtlike than dreamlike. It should be noted that cognitive functioning immediately after abrupt wakening from sleep is likely to carry over some of the characteristics of the preceding sleep state. This phenomenon, known as sleep inertia, can be used as an experimental tool for studying (by inference) cognition during different stages of sleep. For example, studies have shown that stories told in response to thematic apperception test cards (a psychological test used to uncover the dynamics of personality) immediately after awakening from REM sleep are more creative than those told right after non-REM sleep. The combination of confusion, disorientation, cognitive and motor impairment, and amnesia sometimes following abrupt awakening from delta sleep is justifiably termed sleep drunkenness and can be genuinely life-threatening for people such as navy fighter pilots on alert who must fly their highly complicated supersonic jets off an aircraft carrier in the middle of the night within 5 minutes of being awakened. 3 of 12 10/11/2016 8:41 AM Sleep and dreaming - AccessScience from McGraw-Hill Education http://accessscience.com/content/sleep-and-dreaming/628000 After about 90 minutes, the progression of sleep stages is reversed, back through stage 3 and stage 2 to stage 1 again. However, now the EMG shows virtually no activity at all (Fig. 2e), indicating that muscle tone has reached its lowest possible level, and the EOG discloses the occurrence of rapid eye movements—at first only a few at a time, but later in dramatic profusion. This is REM (or active) sleep. Breathing rate and heart rate become more rapid and irregular, and both males and females show signs of sexual arousal (for example, erections and increased vaginal blood flow). Brain metabolic rate increases to levels that typically exceed the waking state average. This state of intense brain activation is normally accompanied by experiences that seem vividly real while they last, but often vanish within seconds of waking. When people are abruptly awakened from REM sleep, 80–90% of the time they recall vivid and sometimes extremely detailed dreams.

While all this activity is happening in the brain, the body remains almost completely still (except for small twitches), because it is temporarily paralyzed during REM sleep to prevent dreams from being acted out. The brainstem system that causes the paralysis of REM sleep does not always inactivate immediately upon awakening. The resulting experience, known as sleep paralysis, can be terrifying, but it is quite harmless and normal if it occurs only occasionally. However, frequent sleep paralysis can be a symptom of a disorder of REM sleep called narcolepsy. See also: Sleep disorders (/content/sleep-disorders /757373)

After a REM period lasting perhaps 5 to 15 minutes, a young adult will typically go back through the preceding cycle stages, dreaming vividly three or four more times during the remainder of the night (Fig. 3), with two major modifications. First, decreasing amounts of slow-wave EEG activity (stages 3 and 4 or delta sleep) occur in each successive cycle. Later in the night, after perhaps the second or third REM period, no delta sleep appears on the EEG at all, only non-REM, stage-2, and REM sleep. Second, as the night proceeds, successive REM periods tend to increase in length, up to a point. While the first REM period commonly lasts less than 10 minutes, later REM periods often last 30 to 40 minutes, and even an hour or more is not exceptionally uncommon late in the sleep cycle. At the same time that the REM periods are getting longer, the intervals between them tend to decrease in length from the approximately 90 minutes characteristic of the first part of the night to as little as 20 or 30 minutes in the late morning. The fact that for humans most REM occurs in the last portion of the sleep cycle as dawn approaches suggests that REM serves a function related to preparation for waking activity.

Fig. 3 Sleep histogram showing sleep cycles during a typical night. Note the increasing amount of REM sleep as the night progresses.

Finally, after four or five periods of dreaming sleep, the sleeper wakes up (for perhaps the tenth time during the night) and gets up for the day. It may be difficult to believe that brief awakenings occur this frequently during an average night; however, they are promptly forgotten. This retrograde amnesia is a normal feature of sleep: information in short-term memory at sleep

4 of 12 10/11/2016 8:41 AM Sleep and dreaming - AccessScience from McGraw-Hill Education http://accessscience.com/content/sleep-and-dreaming/628000 onset is usually not transferred into more permanent storage.

Measuring sleepiness

Probably the most obvious relationship between sleep and wakefulness is the fact that variations in the amount of sleep received predict subsequent variations in alertness the following day or days. Too little sleep one night is likely to result in increased sleepiness and decreased alertness during the following day. Variations in alertness/sleepiness form a continuum of states ranging from hyperexcitation and seizure at one extreme to coma and death at the other. Abnormal or pathological states of excessive activation are accompanied by restless hypervigilance, distractibility, agitation, insomnia, and mania. Optimal levels of activation are accompanied by feelings of energy, interest, and motivation to act. Suboptimal activation is accompanied by decreases in such feelings and motivations, along with varying levels of sleepiness, a specific drive motivating rest, inaction, and sleep. Whether sleepiness is experienced as pleasant or unpleasant depends on whether conditions allow consummation of the desire to sleep.

There are various behavioral indications of sleepiness (for example, yawning, head nodding, eye closure, and performance deficits), as well as subjective self-evaluation scales. For example, the Stanford Sleepiness Scale consists of statements that describe seven states of sleepiness/alertness varying from “wide awake” (1), through “somewhat foggy, let down” (4), to “no longer fighting sleep, sleep onset soon, having dreamlike thoughts” (7). (Of course, “asleep” is off the scale.) Subjective scales are quick, cost-effective, and easy to administer but offer relatively low reliability compared with objective measures.

The “gold standard” for quantifying sleepiness is the Multiple Sleep Latency Test (MSLT). It is based on the assumption that degrees of sleepiness are best measured by the speed with which a person falls asleep under standard conditions. Typically, the MSLT is carried out in a sleep laboratory with subjects wired for sleep recordings (EEG, EOG, and EMG). The MSLT is typically performed in five sessions, starting at 10 a.m. and after every 2 hours thereafter until 6 p.m. Care is taken to ensure that the subject is fully comfortable during each session. After taking care of any requirements for comfort (such as a drink of water and going to the bathroom) in time for the scheduled session, the subject enters a dark, quiet bedroom and the electrodes placed on the subject's body are plugged into the polysomnograph. The subject lies on the bed, and at the scheduled start time is told to close his or her eyes and try to go sleep. Usually, prominent alpha waves are visible in the EEG at first. Sleep onset is scored at the abrupt disappearance of alpha waves, and sleep latency is measured as the time from eyes-shut to sleep onset. Immediately at sleep onset, the subject is promptly awakened and required to get out of bed, preventing the accumulation of more than a few seconds of sleep per trial. If the subject fails to fall asleep within 20 minutes, the trial ends and the sleep latency is scored as the maximum, 20 minutes.

The results from the multiple tests are averaged to provide a single measure of daytime alertness. MSLT scores greater than 15 minutes merit the label “Good Alertness”; scores more than 10 but less than 15 minutes, “Can Get By”; greater than 5 but less than 10 minutes, “Borderline”; and scores less than 5 minutes, “Twilight Zone,” because memory and clarity of thinking are usually substantially impaired with this degree of sleepiness.

The quickest method of rapidly increasing sleepiness is total sleep deprivation. An MSLT study showed that young adults were in the Twilight Zone after a single night of total sleep deprivation. After two 9-hour recovery nights of sleep following two nights of total sleep deprivation, they were out of the Twilight Zone in only half the day's MSLT sessions. The fact that two full nights of sleep were insufficient to return alertness to full wakefulness suggests that a sleep debt has been incurred by the total sleep deprivation. But total sleep deprivation is not the usual source of increased daytime sleep tendency, because even partial sleep deprivation has cumulative effects. For example, after three baseline nights with 9 hours in bed per night, subjects were restricted to 5 hours in bed per night for seven consecutive nights, and the MSLT was administered on each subsequent day. Subjects showed a progressive increase in sleepiness for every night of the study. Subjects' scores dropped

5 of 12 10/11/2016 8:41 AM Sleep and dreaming - AccessScience from McGraw-Hill Education http://accessscience.com/content/sleep-and-dreaming/628000 from Good Alertness to Can Get By after only one night of partial sleep deprivation. By night 5 of partial sleep deprivation, they reached Borderline and would have been in the Twilight Zone by night 10 if the study had continued.

The MSLT also reveals that it is typical that sleepiness reaches a peak in midafternoon. A person can be very sleepy at 4 p.m. but by 6 or 8 p.m. feel quite alert without having received any sleep in the meantime. This is due to another factor determining alertness/sleepiness in addition to immediately prior sleep or wakefulness, namely a circadian rhythm. According to the opponent process model, there are two independent processes involved which together produce the circadian process of sleep and wakefulness. One process, whose function is to induce and maintain sleep, is called sleep homeostasis. The other process, whose function is to induce and maintain alertness, is called clock-dependent alerting.

A variation on the sleep deprivation technique is selective sleep deprivation. By waking subjects whenever they show a particular sleep stage, it is theoretically possible to determine the effects of that particular sleep stage, usually REM sleep. REM deprivation was pioneered by W. C. Dement, and the early studies produced dramatic demonstrations of the need for REM sleep. Later, more carefully controlled studies found rather more modest effects of REM deprivation on personality, cognitive function, and motivation. Probably the most consistent effect is an intensification of drives and a lowering of thresholds for cortical stimulation. What manifests as hypersexuality and hyperphagia in cats shows itself in humans as increased irritability and perhaps some tie-loosening. In fact, it is probably the threshold-lowering effect of REM deprivation that causes a single night of REM deprivation to significantly improve mood in depressed patients. See also: Affective disorders (/content/affective-disorders/013750)

Both total and selective quiet-sleep deprivation will produce corresponding increases of sleep-drive and intensification of delta wave activity (Fig. 2d). After a night of total sleep deprivation, the first few hours of subsequent sleep will be almost exclusively non-REM sleep, suggesting that quiet sleep may be even more functionally important than REM sleep.

Evolution and function of sleep

What selective forces were responsible for the evolution of sleep? The major source of daily change in local conditions on Earth derives from day and night. It is a fact of life that most organisms are adapted to either the dark or light phase of the cycle. Therefore, a biological process that limits activity to the phase of the cycle to which the organism is adapted will enhance survival. Most likely sleep developed out of the rest phase of the BRAC, allowing organisms to minimize interactions with the world during the less favorable phase, while engaging in a variety of “off-line” internal maintenance operations, including growth, repair, synthesis of biochemicals consumed during waking activity, and energy conservation, as well as memory consolidation. The fact that different species have many differences in sleep structure, process, and function fits with the idea that sleep serves the specific adaptive needs of each species. For example, quiet sleep and active sleep have different evolutionary histories and serve different biological functions. Quiet sleep appears to be an older form of sleep with simpler and more universal functions related to energy conservation, growth, and restoration. Active sleep is a mammalian invention with functions that appear to be related to specifically mammalian needs such as live birth. The portion of total sleep composed of REM is at its highest level perinatally: newborn humans spend 8 hours per day in REM sleep, with even more time during the last 6 weeks before birth. The time of maximal REM corresponds to the time of maximal growth of the brain. A number of theorists, including H. Roffwarg and M. Jouvet, suggest that this is not a coincidence but points to the main evolutionary function of REM: to provide a source of endogenous stimulation supporting the unfolding of genetic programming and self-organization of the brain. REM time decreases continuously until puberty, when it reaches the 2 hours or less that it maintains throughout the remainder of life. The fact that REM time does not decrease to zero after the full development of the nervous system suggests secondary adaptive advantages afforded by REM during adulthood, which may include facilitation of difficult learning and preparation of the brain for optimal functioning on arousal. This latter function would account for the unequal distribution of REM throughout the night, with most REM closest to the end of the night. The amount of time spent

6 of 12 10/11/2016 8:41 AM Sleep and dreaming - AccessScience from McGraw-Hill Education http://accessscience.com/content/sleep-and-dreaming/628000 asleep by different species, and especially the amount of time spent in the deep stage of REM, varies widely as a function of lifestyle. For example, predators sleep deeply and long, while prey species sleep little with still less REM. See also: Brain (/content/brain/093200); Mammalia (/content/mammalia/402500)

Both phases of sleep appear to serve a variety of distinct functions related to learning and memory. For example, quiet sleep has been shown to enhance consolidation of declarative memory, while active sleep enhances consolidation of procedural memory.

As mentioned above, the delta activity of NREM sleep is homeostatically regulated, increasing with wakefulness and decreasing during sleep. In 2004, R. Huber, G. Tononi, and colleagues found local enhancement of delta activity in specific brain areas associated with a particular motor-learning task, providing strong support for a cellular need for sleep, as well as a specific role in learning consolidation.

A number of studies have recently established that performance improvements on a variety of perceptual, motor, and cognitive tasks following a night of sleep are specifically due to sleep rather than, say, the passage of time.

Although the modern understanding of sleep and dreaming has had the benefit of half a century of scientific research, there are still no simple and conclusive answers to the questions of why we sleep. The more we learn, the more complex sleep seems.

Dreams

From the biological perspective, the basic task of the brain is to predict and control the organism's actions and regulate those actions to achieve optimal outcomes (in terms of survival and reproduction). To accomplish this task, the brain in some sense internally “models” the world. The waking brain bases the features of its world model primarily on the current information received from the senses and secondarily on expectations derived from past experience.

In contrast, the sleeping brain acquires little information from the senses. Therefore, in sleep, the primary sources of information available to the brain are the current motivational state of the organism (for example, current concerns, fears, hunger, and desires) and past experience (for example, both species-specific instincts and personal memories of recent and associated past experiences of potential relevance). According to this theory, dreams result from brains using internal information to create a simulation of the external world, in a manner directly parallel to the process of waking perception, minus most sensory input. See also: Perception (/content/perception/497500)

Dreaming experience is commonly viewed as qualitatively distinct from waking experience. Dreams are often believed to be characterized by lack of reflection and an inability to act deliberately and with intention. However, this view has not been based on equivalent measurements of waking and dreaming state experiences. To achieve equivalence, it is necessary to evaluate waking experience retrospectively, in the same way that dreams are evaluated. In 1995, S. LaBerge et al. published the results of a study examining cognition in dreaming and waking by sampling recall of recent experiences from both conditions and collecting responses to questions pertaining to thought processes during the period recalled. Each of 167 subjects contributed detailed reports and questionnaires from a randomly selected sample of waking experience and a corresponding sample of dreaming experience. After recording each experience, the subjects answered a series of questions about the experience described, assessing whether or not they had engaged in a number of cognitive and other activities. These included deliberate choice between alternatives, reflection, sudden distraction of attention, focused intention, self-consciousness, and emotion. Differences appeared between waking and dreaming samples for several variables: 49% of subjects reported deliberate choice from dreaming versus 74% from waking; 41% of subjects reported public self-consciousness from dreams versus 30% from waking; and 86% reported emotion from dreams versus 74% from waking.

7 of 12 10/11/2016 8:41 AM Sleep and dreaming - AccessScience from McGraw-Hill Education http://accessscience.com/content/sleep-and-dreaming/628000 It is notable that significant differences between dreaming and waking were not evident for other cognitive activities, and none of the measured cognitive functions were absent or rare in dreams. In terms of the typical, waking and dreaming experiences have much in common, with dreams being characterized by somewhat more emotionality and somewhat less choice.

However, less typical examples reveal a wider range of qualities of consciousness in dreaming than in the usual waking state. For example, at one end of the spectrum, dreams at times exhibit cognitive and perceptual errors similar to those produced by brain damage. However, dream bizarreness also shows the creative recombinatory potential of the dreaming brain. Moreover, as illustrated by lucid dreams, dreaming consciousness can also be as volitional and rational as waking consciousness. See also: Consciousness (/content/consciousness/157500)

Cognizant or lucid dreaming

Typically, people are not explicitly aware of the fact that they are dreaming while they are dreaming; however, at times, a remarkable exception occurs. During such lucid dreams, it is possible to freely remember the circumstances of waking life, to think clearly, and to act deliberately upon reflection or in accordance with plans decided upon before sleep, all while experiencing a dream world that seems vividly real.

Although accounts of lucid dreaming go at least as far back as Aristotle, until recently dream reports of this sort were received with considerable skepticism. In the absence of objective proof, sleep researchers doubted that the dreaming brain was capable of such a high degree of mental functioning and consciousness. A new technique involving eye movement signals, developed independently by researchers in the United States and in England in the late 1970s, proved the reality of lucid dreaming. The technique was based on earlier studies which found that the directions of eye movements recorded during REM sleep sometimes exactly corresponded to the directions in which subjects reported they had been looking in their dreams. It was reasoned that if lucid dreamers can in fact act volitionally, they should be able to prove it by making a prearranged eye movement signal marking the exact time they became lucid. Using this approach, researchers verified reports of lucid dreams from five subjects by eye movement signals. All of the signals, and therefore lucid dreams, had occurred during uninterrupted REM sleep (Fig. 4). This result has been replicated in a number of independent laboratories around the world.

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Fig. 4 Voluntary eye movement signaling marks lucid dreaming during uninterrupted REM sleep. Four channels of physiological data [central EEG (C 3-A 2 ), left and right eye movements (LOC and ROC), and chin muscle tone (EMG)] from the last 8 minutes of a 30-minute REM period are shown. Upon awakening, the subject reported having made five eye movement signals (labeled 1–5). The first signal (1, LRLR) marked the onset of lucidity. Emotional sweating artifacts can be observed in the EEG at this point. During the following 90 seconds, the subject “flew about,” exploring his dream world until he believed he had awakened, at which point he made the signal for awakening (2, LRLRLRLR). After another 90 seconds, the subject realized that he was still dreaming and signaled (3) with three pairs of eye movements. Realizing that this was too many, he correctly signaled with two pairs (4). Finally, upon awakening 100 seconds later, he signaled appropriately (5, LRLRLRLR). Calibrations are 50 μV and 5 seconds.

Lucid dreaming as a method for studying consciousness

The eye movement signaling experiments illustrate an important approach to the study of dreaming consciousness. The attempt to apply rigorous scientific methodology to the study of such phenomena as mental imagery, hallucinations, dreaming, and conscious processes in general faces a major challenge: The most direct account available of the private events occurring in a person's mind is that individual's subjective report. However, subjective reports are difficult to verify objectively, and introspection is far from being an unbiased or direct process of observation. There are two strategies likely to increase the reliability of subjective reports: (1) the use of highly trained subjects (in the context of dream research, this is best achieved with lucid dreamers) who are skillful reporters of consciousness, and (2) the use of a psychophysiological approach to provide validation of subjective reports by correlating them with physiological measures. See also: Hallucination (/content/hallucination/900193)

Using these approaches in a series of studies, researchers found that various dreamed experiences (including time estimation, breathing, singing, counting, and sexual activity) produce effects on the dreamer's brain (and to a lesser extent, body) remarkably similar to the physiological effects that are produced by actual experiences of the corresponding events while awake. The results of these studies support the following picture: During REM dreaming, the events that a person consciously experiences (or seems to experience) are the results of patterns of central nervous system activity that produce, in turn, effects on the autonomic nervous system and that are body-isomorphic (similar in appearance) to the effects that would occur if a person actually experienced the corresponding events while awake. This explains, in part, why people

9 of 12 10/11/2016 8:41 AM Sleep and dreaming - AccessScience from McGraw-Hill Education http://accessscience.com/content/sleep-and-dreaming/628000 regularly mistake dreams for reality. To the functional systems of neuronal activity that construct the experiential world model of consciousness, dreaming of perceiving or doing something is equivalent to actually perceiving or doing it.

Dream recall

The average person remembers a dream only once or twice a week. Given the fact that every person dreams each night, that leaves at least 95% of most dreams forgotten (in REM alone; assuming five REM periods per night with perhaps two dreams per REM period). A variety of theories have been put forward suggesting often fanciful explanations of why dreams are so easily forgotten, ranging from Sigmund Freud's belief that dreams are repressed because they contain so much taboo dream thought, to F. Crick's view that the content of dreams is what the brain is trying to unlearn, and therefore ought not to be remembered. Of course, standard memory theory explains much of what is remembered of dreams and what is not, which aspects of dream content are more readily recalled and which aspects are not, and so on. See also: Memory (/content /memory/414300); Psychoanalysis (/content/psychoanalysis/554400)

But why are dreams so much more difficult to recall than waking experiences? Evolution may provide the answer. As S. LaBerge reasoned in 1985, humans learn that dreams are distinct from other experiences by talking with other humans. Nonspeaking animals, however, have no way to tell each other how to distinguish dreams from reality. Explicit dream recall would thus be disorienting and maladaptive for all the nonlinguistic mammals having REM sleep during the 140 million years since REM evolved. The fact that explicit dream recall obviously is maladaptive for cats, dogs, bats, whales, and all of the rest of the mammalian dreamers except humans very likely explains why dreams are difficult for humans to recall. The dreams may be so, according to this view, because of natural selection. Like all mammals, humans and their ancestors might have been protected from dangerous confusion by the evolution of mechanisms that made forgetting dreams the normal course of affairs. See also: Organic evolution (/content/organic-evolution/475150)

Stephen LaBerge

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Additional Readings

J. Gackenbach and S. LaBerge (eds.), Conscious Mind, Sleeping Brain, Plenum Press, New York, 1988

M. H. Kryger, T. Roth, and W. C. Dement (eds.), Principles and Practice of Sleep Medicine, 5th ed., Saunders, Philadelphia, 2010

S. LaBerge and D. J. DeGracia, Varieties of lucid dreaming experience, in R. G. Kunzendorf and B. Wallace (eds.), Individual Differences in Conscious Experience, pp. 269–307, John Benjamins, Amsterdam, 2000

S. LaBerge and H. Rheingold, Exploring the World of Lucid Dreaming, Ballantine, New York, 1990

R. L. Van de Castle, Our Dreaming Mind, Ballantine, New York, 1994

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