A dream is a succession of images, ideas, emotions, and sensations that usually occur involuntarily in the mind during certain stages of sleep.[1]
The content and purpose of dreams are not fully understood, though they
have been a topic of scientific, philosophical and religious interest
throughout recorded history. Dream interpretation
is the attempt at drawing meaning from dreams and searching for an
underlying message. The scientific study of dreams is called oneirology.[2]
Dreams mainly occur in the rapid-eye movement (REM) stage of sleep—when brain activity is high and resembles that of being awake. REM sleep is revealed by continuous movements of the eyes during sleep. At times, dreams may occur during other stages of sleep. However, these dreams tend to be much less vivid or memorable.[3] The length of a dream can vary; they may last for a few seconds, or approximately 20–30 minutes.[3] People are more likely to remember the dream if they are awakened during the REM phase. The average person has three to five dreams per night, and some may have up to seven;[4] however, most dreams are immediately or quickly forgotten.[5] Dreams tend to last longer as the night progresses. During a full eight-hour night sleep, most dreams occur in the typical two hours of REM.[6]
Opinions about the meaning of dreams have varied and shifted through time and culture. Many endorse the Freudian theory of dreams – that dreams reveal insight into hidden desires and emotions. Other prominent theories include those suggesting that dreams assist in memory formation, problem solving, or simply are a product of random brain activation.[7] Blechner[8] proposed that dreams show how the mind engages in thought when not required to produce communicable versions of thought. The earliest recorded dreams were acquired from materials dating back approximately 5000 years, in Mesopotamia, where they were documented on clay tablets. In the Greek and Roman periods, the people believed that dreams were direct messages from deities or deceased persons, and that they predicted the future. Some cultures practiced dream incubation with the intention of cultivating dreams that are of prophecy.[9]
Sigmund Freud, who developed the psychological discipline of psychoanalysis, wrote extensively about dream theories and their interpretations in the early 1900s.[10] He explained dreams as manifestations of one's deepest desires and anxieties, often relating to repressed childhood memories or obsessions. Furthermore, he believed that virtually every dream topic, regardless of its content, represented the release of sexual tension.[11] In The Interpretation of Dreams (1899), Freud developed a psychological technique to interpret dreams and devised a series of guidelines to understand the symbols and motifs that appear in our dreams. In modern times, dreams have been seen as a connection to the unconscious mind. They range from normal and ordinary to overly surreal and bizarre. Dreams can have varying natures, such as being frightening, exciting, magical, melancholic, adventurous, or sexual. The events in dreams are generally outside the control of the dreamer, with the exception of lucid dreaming, where the dreamer is self-aware.[12] Dreams can at times make a creative thought occur to the person or give a sense of inspiration.
In Chinese history, people wrote of two vital aspects of the soul of
which one is freed from the body during slumber to journey in a dream
realm, while the other remained in the body,[20] although this belief and dream interpretation had been questioned since early times, such as by the philosopher Wang Chong (27–97 AD).[20] The Indian text Upanishads,
written between 900 and 500 BC, emphasize two meanings of dreams. The
first says that dreams are merely expressions of inner desires. The
second is the belief of the soul leaving the body and being guided until
awakened.
The Greeks shared their beliefs with the Egyptians on how to interpret good and bad dreams, and the idea of incubating dreams. Morpheus, the Greek god of dreams, also sent warnings and prophecies to those who slept at shrines and temples. The earliest Greek beliefs about dreams were that their gods physically visited the dreamers, where they entered through a keyhole, exiting the same way after the divine message was given.
Antiphon wrote the first known Greek book on dreams in the 5th century BC. In that century, other cultures influenced Greeks to develop the belief that souls left the sleeping body.[21] Hippocrates (469–399 BC) had a simple dream theory: during the day, the soul receives images; during the night, it produces images. Greek philosopher Aristotle (384–322 BC) believed dreams caused physiological activity. He thought dreams could analyze illness and predict diseases. Marcus Tullius Cicero, for his part, believed that all dreams are produced by thoughts and conversations a dreamer had during the preceding days.[22] Cicero's Somnium Scipionis described a lengthy dream vision, which in turn was commented on by Macrobius in his Commentarii in Somnium Scipionis.
Herodotus in his The Histories, writes "The visions that occur to us in dreams are, more often than not, the things we have been concerned about during the day."[23]
In Welsh history, The Dream of Rhonabwy (Welsh: Breuddwyd Rhonabwy) is a Middle Welsh prose tale. Set during the reign of Madog ap Maredudd, prince of Powys (died 1160), it is dated to the late 12th or 13th century. It survives in only one manuscript, the Red Book of Hergest, and has been associated with the Mabinogion since its publication by Lady Charlotte Guest in the 19th century. The bulk of the narrative describes a dream vision experienced by its central character, Rhonabwy, a retainer of Madog, in which he visits the time of King Arthur.[24]
Also in Welsh history, the tale 'The Dream of Macsen Wledig' is a romanticised story about the Roman emperor Magnus Maximus, called Macsen Wledig in Welsh. Born in Hispania, he became a legionary commander in Britain, assembled a Celtic army and assumed the title of Emperor of the Western Roman Empire in 383. He was defeated in battle in 385 and beheaded at the direction of the Eastern Roman emperor.[25]
However, research by Mark Solms suggests that dreams are generated in the forebrain, and that REM sleep and dreaming are not directly related.[63] While working in the neurosurgery department at hospitals in Johannesburg and London, Solms had access to patients with various brain injuries. He began to question patients about their dreams and confirmed that patients with damage to the parietal lobe stopped dreaming; this finding was in line with Hobson's 1977 theory. However, Solms did not encounter cases of loss of dreaming with patients having brainstem damage. This observation forced him to question Hobson's prevailing theory, which marked the brainstem as the source of the signals interpreted as dreams.
Zhang assumes that during REM sleep the unconscious part of a brain is busy processing the procedural memory; meanwhile, the level of activation in the conscious part of the brain descends to a very low level as the inputs from the sensory systems are basically disconnected. This triggers the "continual-activation" mechanism to generate a data stream from the memory stores to flow through the conscious part of the brain. Zhang suggests that this pulse-like brain activation is the inducer of each dream. He proposes that, with the involvement of the brain associative thinking system, dreaming is, thereafter, self-maintained with the dreamer's own thinking until the next pulse of memory insertion. This explains why dreams have both characteristics of continuity (within a dream) and sudden changes (between two dreams).[64][65] A detailed explanation of how a dream is synthesized is given in a later paper.[66]
A 2001 study showed evidence that illogical locations, characters,
and dream flow may help the brain strengthen the linking and
consolidation of semantic memories.[68] These conditions may occur because, during REM sleep, the flow of information between the hippocampus and neocortex is reduced.[69]
Increasing levels of the stress hormone cortisol late in sleep (often during REM sleep) causes this decreased communication. One stage of memory consolidation is the linking of distant but related memories. Payne and Nadal hypothesize these memories are then consolidated into a smooth narrative, similar to a process that happens when memories are created under stress.[70] Robert (1886),[71] a physician from Hamburg, was the first who suggested that dreams are a need and that they have the function to erase (a) sensory impressions that were not fully worked up, and (b) ideas that were not fully developed during the day. By the dream work, incomplete material is either removed (suppressed) or deepened and included into memory. Robert's ideas were cited repeatedly by Freud in his Die Traumdeutung. Hughlings Jackson (1911) viewed that sleep serves to sweep away unnecessary memories and connections from the day.
This was revised in 1983 by Crick and Mitchison's "reverse learning" theory, which states that dreams are like the cleaning-up operations of computers when they are off-line, removing (suppressing) parasitic nodes and other "junk" from the mind during sleep.[72][73] However, the opposite view that dreaming has an information handling, memory-consolidating function (Hennevin and Leconte, 1971) is also common.
Hobson proposed the activation-synthesis theory, which states that "there is a randomness of dream imagery and the randomness synthesizes dream-generated images to fit the patterns of internally generated stimulations".[78] This theory is based on the physiology of REM sleep, and Hobson believes dreams are the outcome of the forebrain reacting to random activity beginning at the brainstem. The activation-synthesis theory hypothesizes that the peculiar nature of dreams is attributed to certain parts of the brain trying to piece together a story out of what is essentially bizarre information.[79]
However, evolutionary psychologists believe dreams serve some adaptive function for survival. Deirdre Barrett describes dreaming as simply "thinking in different biochemical state" and believes people continue to work on all the same problems—personal and objective—in that state.[80] Her research finds that anything—math, musical composition, business dilemmas—may get solved during dreaming.[81][82] In a related theory, which Mark Blechner terms "Oneiric Darwinism," dreams are seen as creating new ideas through the generation of random thought mutations. Some of these may be rejected by the mind as useless, while others may be seen as valuable and retained.[83]
Finnish psychologist Antti Revonsuo posits that dreams have evolved for "threat simulation" exclusively. According to the Threat Simulation Theory he proposes, during much of human evolution physical and interpersonal threats were serious, giving reproductive advantage to those who survived them. Therefore, dreaming evolved to replicate these threats and continually practice dealing with them. In support of this theory, Revonsuo shows that contemporary dreams comprise much more threatening events than people meet in daily non-dream life, and the dreamer usually engages appropriately with them.[84] It is suggested by this theory that dreams serve the purpose of allowing for the rehearsal of threatening scenarios in order to better prepare an individual for real-life threats.
According to Tsoukalas (2012) the biology of dreaming is related to the reactive patterns elicited by predatorial encounters (especially the tonic immobility reflex), a fact that lends support to evolutionary theories claiming that dreams specialize in threat avoidance or emotional processing.[61]
Dreams mainly occur in the rapid-eye movement (REM) stage of sleep—when brain activity is high and resembles that of being awake. REM sleep is revealed by continuous movements of the eyes during sleep. At times, dreams may occur during other stages of sleep. However, these dreams tend to be much less vivid or memorable.[3] The length of a dream can vary; they may last for a few seconds, or approximately 20–30 minutes.[3] People are more likely to remember the dream if they are awakened during the REM phase. The average person has three to five dreams per night, and some may have up to seven;[4] however, most dreams are immediately or quickly forgotten.[5] Dreams tend to last longer as the night progresses. During a full eight-hour night sleep, most dreams occur in the typical two hours of REM.[6]
Opinions about the meaning of dreams have varied and shifted through time and culture. Many endorse the Freudian theory of dreams – that dreams reveal insight into hidden desires and emotions. Other prominent theories include those suggesting that dreams assist in memory formation, problem solving, or simply are a product of random brain activation.[7] Blechner[8] proposed that dreams show how the mind engages in thought when not required to produce communicable versions of thought. The earliest recorded dreams were acquired from materials dating back approximately 5000 years, in Mesopotamia, where they were documented on clay tablets. In the Greek and Roman periods, the people believed that dreams were direct messages from deities or deceased persons, and that they predicted the future. Some cultures practiced dream incubation with the intention of cultivating dreams that are of prophecy.[9]
Sigmund Freud, who developed the psychological discipline of psychoanalysis, wrote extensively about dream theories and their interpretations in the early 1900s.[10] He explained dreams as manifestations of one's deepest desires and anxieties, often relating to repressed childhood memories or obsessions. Furthermore, he believed that virtually every dream topic, regardless of its content, represented the release of sexual tension.[11] In The Interpretation of Dreams (1899), Freud developed a psychological technique to interpret dreams and devised a series of guidelines to understand the symbols and motifs that appear in our dreams. In modern times, dreams have been seen as a connection to the unconscious mind. They range from normal and ordinary to overly surreal and bizarre. Dreams can have varying natures, such as being frightening, exciting, magical, melancholic, adventurous, or sexual. The events in dreams are generally outside the control of the dreamer, with the exception of lucid dreaming, where the dreamer is self-aware.[12] Dreams can at times make a creative thought occur to the person or give a sense of inspiration.
Classical history
Dreaming of the Tiger Spring (虎跑夢泉)
The Greeks shared their beliefs with the Egyptians on how to interpret good and bad dreams, and the idea of incubating dreams. Morpheus, the Greek god of dreams, also sent warnings and prophecies to those who slept at shrines and temples. The earliest Greek beliefs about dreams were that their gods physically visited the dreamers, where they entered through a keyhole, exiting the same way after the divine message was given.
Antiphon wrote the first known Greek book on dreams in the 5th century BC. In that century, other cultures influenced Greeks to develop the belief that souls left the sleeping body.[21] Hippocrates (469–399 BC) had a simple dream theory: during the day, the soul receives images; during the night, it produces images. Greek philosopher Aristotle (384–322 BC) believed dreams caused physiological activity. He thought dreams could analyze illness and predict diseases. Marcus Tullius Cicero, for his part, believed that all dreams are produced by thoughts and conversations a dreamer had during the preceding days.[22] Cicero's Somnium Scipionis described a lengthy dream vision, which in turn was commented on by Macrobius in his Commentarii in Somnium Scipionis.
Herodotus in his The Histories, writes "The visions that occur to us in dreams are, more often than not, the things we have been concerned about during the day."[23]
Beginning of "The Dream of Macsen Wledig" from the White Book of Rhydderch, f.45.r
Also in Welsh history, the tale 'The Dream of Macsen Wledig' is a romanticised story about the Roman emperor Magnus Maximus, called Macsen Wledig in Welsh. Born in Hispania, he became a legionary commander in Britain, assembled a Celtic army and assumed the title of Emperor of the Western Roman Empire in 383. He was defeated in battle in 385 and beheaded at the direction of the Eastern Roman emperor.[25]
Neurological theories
Activation synthesis theory
In 1976 J. Allan Hobson and Robert McCarley proposed a new theory that changed dream research, challenging the previously held Freudian view of dreams as unconscious wishes to be interpreted. They assume that the same structures that induce REM sleep also generate sensory information. Hobson's 1976 research suggested that the signals interpreted as dreams originate in the brainstem during REM sleep. According to Hobson and other researchers, circuits in the brainstem are activated during REM sleep. Once these circuits are activated, areas of the limbic system involved in emotions, sensations, and memories, including the amygdala and hippocampus, become active. The brain synthesizes and interprets these activities; for example, changes in the physical environment such as temperature and humidity, or physical stimuli such as ejaculation, and attempts to create meaning from these signals, result in dreaming.However, research by Mark Solms suggests that dreams are generated in the forebrain, and that REM sleep and dreaming are not directly related.[63] While working in the neurosurgery department at hospitals in Johannesburg and London, Solms had access to patients with various brain injuries. He began to question patients about their dreams and confirmed that patients with damage to the parietal lobe stopped dreaming; this finding was in line with Hobson's 1977 theory. However, Solms did not encounter cases of loss of dreaming with patients having brainstem damage. This observation forced him to question Hobson's prevailing theory, which marked the brainstem as the source of the signals interpreted as dreams.
Continual-activation theory
Combining Hobson's activation synthesis hypothesis with Solms' findings, the continual-activation theory of dreaming presented by Jie Zhang proposes that dreaming is a result of brain activation and synthesis; at the same time, dreaming and REM sleep are controlled by different brain mechanisms. Zhang hypothesizes that the function of sleep is to process, encode, and transfer the data from the temporary memory store to the long-term memory store. During NREM sleep the conscious-related memory (declarative memory) is processed, and during REM sleep the unconscious related memory (procedural memory) is processed.[64]Zhang assumes that during REM sleep the unconscious part of a brain is busy processing the procedural memory; meanwhile, the level of activation in the conscious part of the brain descends to a very low level as the inputs from the sensory systems are basically disconnected. This triggers the "continual-activation" mechanism to generate a data stream from the memory stores to flow through the conscious part of the brain. Zhang suggests that this pulse-like brain activation is the inducer of each dream. He proposes that, with the involvement of the brain associative thinking system, dreaming is, thereafter, self-maintained with the dreamer's own thinking until the next pulse of memory insertion. This explains why dreams have both characteristics of continuity (within a dream) and sudden changes (between two dreams).[64][65] A detailed explanation of how a dream is synthesized is given in a later paper.[66]
Defensive immobilization: the precursor
According to Tsoukalas (2012) REM sleep is an evolutionary transformation of a well-known defensive mechanism, the tonic immobility reflex. This reflex, also known as animal hypnosis or death feigning, functions as the last line of defense against an attacking predator and consists of the total immobilization of the animal: the animal appears dead (cf. "playing possum"). Tsoukalas claims that the neurophysiology and phenomenology of this reaction shows striking similarities to REM sleep, a fact that suggests a deep evolutionary kinship. For example, both reactions exhibit brainstem control, paralysis, sympathetic activation, and thermoregulatory changes. Tsoukalas claims that this theory integrates many earlier findings into a unified framework.[61][62]As excitations of long-term memory
Eugen Tarnow suggests that dreams are ever-present excitations of long-term memory, even during waking life. The strangeness of dreams is due to the format of long-term memory, reminiscent of Penfield & Rasmussen's findings that electrical excitations of the cortex give rise to experiences similar to dreams. During waking life an executive function interprets long-term memory consistent with reality checking. Tarnow's theory is a reworking of Freud's theory of dreams in which Freud's unconscious is replaced with the long-term memory system and Freud's "Dream Work" describes the structure of long-term memory.[67]Role in strengthening semantic memories
Location of the hippocampi
Increasing levels of the stress hormone cortisol late in sleep (often during REM sleep) causes this decreased communication. One stage of memory consolidation is the linking of distant but related memories. Payne and Nadal hypothesize these memories are then consolidated into a smooth narrative, similar to a process that happens when memories are created under stress.[70] Robert (1886),[71] a physician from Hamburg, was the first who suggested that dreams are a need and that they have the function to erase (a) sensory impressions that were not fully worked up, and (b) ideas that were not fully developed during the day. By the dream work, incomplete material is either removed (suppressed) or deepened and included into memory. Robert's ideas were cited repeatedly by Freud in his Die Traumdeutung. Hughlings Jackson (1911) viewed that sleep serves to sweep away unnecessary memories and connections from the day.
This was revised in 1983 by Crick and Mitchison's "reverse learning" theory, which states that dreams are like the cleaning-up operations of computers when they are off-line, removing (suppressing) parasitic nodes and other "junk" from the mind during sleep.[72][73] However, the opposite view that dreaming has an information handling, memory-consolidating function (Hennevin and Leconte, 1971) is also common.
Psychological theories
Role in testing and selecting mental schemas
Coutts[74] describes dreams as playing a central role in a two-phase sleep process that improves the mind's ability to meet human needs during wakefulness. During the accommodation phase, mental schemas self-modify by incorporating dream themes. During the emotional selection phase, dreams test prior schema accommodations. Those that appear adaptive are retained, while those that appear maladaptive are culled. The cycle maps to the sleep cycle, repeating several times during a typical night's sleep. Alfred Adler suggested that dreams are often emotional preparations for solving problems, intoxicating an individual away from common sense toward private logic. The residual dream feelings may either reinforce or inhibit contemplated action.Evolutionary psychology theories
Numerous theories state that dreaming is a random by-product of REM sleep physiology and that it does not serve any natural purpose.[75] Flanagan claims that "dreams are evolutionary epiphenomena" and they have no adaptive function. "Dreaming came along as a free ride on a system designed to think and to sleep."[76] Hobson, for different reasons, also considers dreams epiphenomena. He believes that the substance of dreams have no significant influence on waking actions, and most people go about their daily lives perfectly well without remembering their dreams.[77]Hobson proposed the activation-synthesis theory, which states that "there is a randomness of dream imagery and the randomness synthesizes dream-generated images to fit the patterns of internally generated stimulations".[78] This theory is based on the physiology of REM sleep, and Hobson believes dreams are the outcome of the forebrain reacting to random activity beginning at the brainstem. The activation-synthesis theory hypothesizes that the peculiar nature of dreams is attributed to certain parts of the brain trying to piece together a story out of what is essentially bizarre information.[79]
However, evolutionary psychologists believe dreams serve some adaptive function for survival. Deirdre Barrett describes dreaming as simply "thinking in different biochemical state" and believes people continue to work on all the same problems—personal and objective—in that state.[80] Her research finds that anything—math, musical composition, business dilemmas—may get solved during dreaming.[81][82] In a related theory, which Mark Blechner terms "Oneiric Darwinism," dreams are seen as creating new ideas through the generation of random thought mutations. Some of these may be rejected by the mind as useless, while others may be seen as valuable and retained.[83]
Finnish psychologist Antti Revonsuo posits that dreams have evolved for "threat simulation" exclusively. According to the Threat Simulation Theory he proposes, during much of human evolution physical and interpersonal threats were serious, giving reproductive advantage to those who survived them. Therefore, dreaming evolved to replicate these threats and continually practice dealing with them. In support of this theory, Revonsuo shows that contemporary dreams comprise much more threatening events than people meet in daily non-dream life, and the dreamer usually engages appropriately with them.[84] It is suggested by this theory that dreams serve the purpose of allowing for the rehearsal of threatening scenarios in order to better prepare an individual for real-life threats.
According to Tsoukalas (2012) the biology of dreaming is related to the reactive patterns elicited by predatorial encounters (especially the tonic immobility reflex), a fact that lends support to evolutionary theories claiming that dreams specialize in threat avoidance or emotional processing.[61]
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