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A dream is a succession of images, ideas, emotions, and sensations that usually occur involuntarily in the mind during certain stages of sleep.[1] Humans spend about two hours dreaming per night,[2] and each dream lasts around 5 to 20 minutes, although the dreamer may perceive the dream as being much longer than this.[3]
The content and function of dreams have been topics of scientific, philosophical and religious interest throughout recorded history. Dream interpretation, practiced by the Babylonians in the third millennium BCE[4] and even earlier by the ancient Sumerians,[5][6] figures prominently in religious texts in several traditions, and has played a lead role in psychotherapy.[7][8] The scientific study of dreams is called oneirology.[9] Most modern dream study focuses on the neurophysiology of dreams and on proposing and testing hypotheses regarding dream function. It is not known where in the brain dreams originate, if there is a single origin for dreams or if multiple regions of the brain are involved, or what the purpose of dreaming is for the body or mind.
The human dream experience and what to make of it has undergone sizable shifts over the course of history.[10][11] Long ago, according to writings from Mesopotamia and Ancient Egypt, dreams dictated post-dream behaviors to an extent that was sharply reduced in later millennia.[clarification needed] These ancient writings about dreams highlight visitation dreams, where a dream figure, usually a deity or a prominent forebear, commands the dreamer to take specific actions, and which may predict future events.[12][13][14] Framing the dream experience varies across cultures as well as through time.
Dreaming and sleep are intertwined. Dreams occur mainly in the rapid-eye movement (REM) stage of sleep—when brain activity is high and resembles that of being awake. Because REM sleep is detectable in many species, and because research suggests that all mammals experience REM,[15] linking dreams to REM sleep has led to conjectures that animals dream. However, humans dream during non-REM sleep, also, and not all REM awakenings elicit dream reports.[16] To be studied, a dream must first be reduced to a verbal report, which is an account of the subject's memory of the dream, not the subject's dream experience itself. So, dreaming by non-humans is currently unprovable, as is dreaming by human fetuses and pre-verbal infants.[17]
Subjective experience and content[edit]
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Preserved writings from early Mediterranean civilizations indicate a relatively abrupt change in subjective dream experience between Bronze Age antiquity and the beginnings of the classical era.[18]
In visitation dreams reported in ancient writings, dreamers were largely passive in their dreams, and visual content served primarily to frame authoritative auditory messaging.[19][10][20] Gudea, the king of the Sumerian city-state of Lagash (reigned c. 2144–2124 BCE), rebuilt the temple of Ningirsu as the result of a dream in which he was told to do so.[6] After antiquity, the passive hearing of visitation dreams largely gave way to visualized narratives in which the dreamer becomes a character who actively participates.
From the 1940s to 1985, Calvin S. Hall collected more than 50,000 dream reports at Western Reserve University. In 1966, Hall and Robert Van de Castle published The Content Analysis of Dreams, in which they outlined a coding system to study 1,000 dream reports from college students.[21] Results indicated that participants from varying parts of the world demonstrated similarity in their dream content. The only residue of antiquity's authoritative dream figure in the Hall and Van de Castle listing of dream characters is the inclusion of God in the category of prominent persons.[22] Hall's complete dream reports were made publicly available in the mid-1990s by his protégé William Domhoff. More recent studies of dream reports, while providing more detail, continue to cite the Hall study favorably.[23]
In the Hall study, the most common emotion experienced in dreams was anxiety. Other emotions included abandonment, anger, fear, joy, and happiness. Negative emotions were much more common than positive ones.[21] The Hall data analysis showed that sexual dreams occur no more than 10% of the time and are more prevalent in young to mid-teens.[21] Another study showed that 8% of both men's and women's dreams have sexual content.[24] In some cases, sexual dreams may result in orgasms or nocturnal emissions. These are colloquially known as "wet dreams".[25]
The visual nature of dreams is generally highly phantasmagoric; that is, different locations and objects continuously blend into each other. The visuals (including locations, people, and objects) are generally reflective of a person's memories and experiences, but conversation can take on highly exaggerated and bizarre forms. Some dreams may even tell elaborate stories wherein the dreamer enters entirely new, complex worlds and awakes with ideas, thoughts and feelings never experienced prior to the dream.
People who are blind from birth do not have visual dreams. Their dream contents are related to other senses, such as hearing, touch, smell, and taste, whichever are present since birth.[26]
Neurophysiology[edit]
Dream study is popular with scientists exploring the mind–brain problem. Some "propose to reduce aspects of dream phenomenology to neurobiology."[27] But current science cannot specify dream physiology in detail. Protocols in most nations restrict human brain research to non-invasive procedures. In the United States, invasive brain procedures with a human subject are allowed only when these are deemed necessary in surgical treatment to address medical needs of the same human subject.[28] Non-invasive measures of brain activity like electroencephalogram (EEG) voltage averaging or cerebral blood flow cannot identify small but influential neuronal populations.[29] Also, fMRI signals are too slow to explain how brains compute in real time.[30]
Scientists researching some brain functions can work around current restrictions by examining animal subjects. As stated by the Society for Neuroscience, "Because no adequate alternatives exist, much of this research must [sic] be done on animal subjects."[31] However, since animal dreaming can be only inferred, not confirmed, animal studies yield no hard facts to illuminate the neurophysiology of dreams. Examining human subjects with brain lesions can provide clues, but the lesion method cannot discriminate between the effects of destruction and disconnection and cannot target specific neuronal groups in heterogeneous regions like the brain stem.[29]
Generation[edit]
Denied precision tools and obliged to depend on imaging, much dream research has succumbed to the law of the instrument. Studies detect an increase of blood flow in a specific brain region and then credit that region with a role in generating dreams. But pooling study results has led to the newer conclusion that dreaming involves large numbers of regions and pathways, which likely are different for different dream events.[32]
Image creation in the brain involves significant neural activity downstream from eye intake, and it is theorized that "the visual imagery of dreams is produced by activation during sleep of the same structures that generate complex visual imagery in waking perception."[33]
Dreams present a running narrative rather than exclusively visual imagery. Following their work with split-brain subjects, Gazzaniga and LeDoux postulated, without attempting to specify the neural mechanisms, a "left-brain interpreter" that seeks to create a plausible narrative from whatever electro-chemical signals reach the brain's left hemisphere. Sleep research has determined that some brain regions fully active during waking are, during REM sleep, activated only in a partial or fragmentary way.[34] Drawing on this knowledge, textbook author James W. Kalat explains, "[A] dream represents the brain's effort to make sense of sparse and distorted information.... The cortex combines this haphazard input with whatever other activity was already occurring and does its best to synthesize a story that makes sense of the information."[35] Neuroscientist Indre Viskontas is even more blunt, calling often bizarre dream content "just the result of your interpreter trying to create a story out of random neural signaling."[36]
Theories on function[edit]
For many humans across multiple eras and cultures, dreams are believed to have functioned as revealers of truths sourced during sleep from gods or other external entities.[37] Ancient Egyptians believed that dreams were the best way to receive divine revelation, and thus they would induce (or "incubate") dreams. They went to sanctuaries and slept on special "dream beds" in hope of receiving advice, comfort, or healing from the gods.[14] From a Darwinian perspective dreams would have to fulfill some kind of biological requirement, provide some benefit for natural selection to take place, or at least have no negative impact on fitness. Robert (1886),[38] 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. In dreams, incomplete material is either removed (suppressed) or deepened and included into memory. Freud, whose dream studies focused on interpreting dreams, not explaining how or why humans dream, disputed Robert's hypothesis[39] and proposed that dreams preserve sleep by representing as fulfilled those wishes that otherwise would awaken the dreamer.[40] Freud wrote that dreams "serve the purpose of prolonging sleep instead of waking up. Dreams are the GUARDIANS of sleep and not its disturbers."[41]
A turning point in theorizing about dream function came in 1953, when Science published the Aserinsky and Kleitman paper[42] establishing REM sleep as a distinct phase of sleep and linking dreams to REM sleep.[43] Until and even after publication of the Solms 2000 paper that certified the separability of REM sleep and dream phenomena,[16] many studies purporting to uncover the function of dreams have in fact been studying not dreams but measurable REM sleep.
Theories of dream function since the identification of REM sleep include:
Hobson's and McCarley's 1977 activation-synthesis hypothesis, which proposed "a functional role for dreaming sleep in promoting some aspect of the learning process...."[44] In 2010 a Harvard study was published showing experimental evidence that dreams were correlated with improved learning.[45]
Crick's and Mitchison's 1983 "reverse learning" theory, which states that dreams are like the cleaning-up operations of computers when they are offline, removing (suppressing) parasitic nodes and other "junk" from the mind during sleep.[46][47]
Hartmann's 1995 proposal that dreams serve a "quasi-therapeutic" function, enabling the dreamer to process trauma in a safe place.[48]
Revonsuo's 2000 threat simulation hypothesis, whose premise is that during much of human evolution, physical and interpersonal threats were serious, giving reproductive advantage to those who survived them. Dreaming aided survival by replicating these threats and providing the dreamer with practice in dealing with them.[49] In 2015, Revonsuo proposed social simulation theory, which describes dreams as a simulation for training social skills and bonds.[50]
Eagleman's and Vaughn's 2021 defensive activation theory, which says that, given the brain's neuroplasticity, dreams evolved as a visual hallucinatory activity during sleep's extended periods of darkness, busying the occipital lobe and thereby protecting it from possible appropriation by other, non-vision, sense operations.[51]
Erik Hoel proposes, based on artificial neural networks, that dreams prevent overfitting to past experiences; that is, they enable the dreamer to learn from novel situations.[52][53]
Religious and other cultural contexts[edit]
Dreams figure prominently in major world religions. The dream experience for early humans, according to one interpretation, gave rise to the notion of a human "soul,"[54] a central element in much religious thought. J. W. Dunne wrote:
But there can be no reasonable doubt that the idea of a soul must have first arisen in the mind of primitive man as a result of observation of his dreams. Ignorant as he was, he could have come to no other conclusion but that, in dreams, he left his sleeping body in one universe and went wandering off into another. It is considered that, but for that savage, the idea of such a thing as a 'soul' would never have even occurred to mankind....[55]
Hindu[edit]
In the Mandukya Upanishad, part of the Veda scriptures of Indian Hinduism, a dream is one of three states that the soul experiences during its lifetime, the other two states being the waking state and the sleep state.[56] The earliest Upanishads, written before 300 BCE, 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.
Abrahamic[edit]
In Judaism, dreams are considered part of the experience of the world that can be interpreted and from which lessons can be garnered. It is discussed in the Talmud, Tractate Berachot 55–60.
The ancient Hebrews connected their dreams heavily with their religion, though the Hebrews were monotheistic and believed that dreams were the voice of one God alone. Hebrews also differentiated between good dreams (from God) and bad dreams (from evil spirits). The Hebrews, like many other ancient cultures, incubated dreams in order to receive a divine revelation. For example, the Hebrew prophet Samuel would "lie down and sleep in the temple at Shiloh before the Ark and receive the word of the Lord", and Joseph interpreted a Pharaoh's dream of seven lean cows swallowing seven fat cows as meaning the subsequent seven years would be bountiful, followed by seven years of famine. Most of the dreams in the Bible are in the Book of Genesis.[57]
Christians mostly shared the beliefs of the Hebrews and thought that dreams were of a supernatural character because the Old Testament includes frequent stories of dreams with divine inspiration. The most famous of these dream stories was Jacob's dream of a ladder that stretches from Earth to Heaven. Many Christians preach that God can speak to people through their dreams. The famous glossary, the Somniale Danielis, written in the name of Daniel, attempted to teach Christian populations to interpret their dreams.
Iain R. Edgar has researched the role of dreams in Islam.[58] He has argued that dreams play an important role in the history of Islam and the lives of Muslims, since dream interpretation is the only way that Muslims can receive revelations from God since the death of the last prophet, Muhammad.[59] According to Edgar, Islam classifies three types of dreams. Firstly, there is the true dream (al-ru’ya), then the false dream, which may come from the devil (shaytan), and finally, the meaningless everyday dream (hulm). This last dream could be brought forth by the dreamer's ego or base appetite based on what they experienced in the real world. The true dream is often indicated by Islam's hadith tradition.[59] In one narration by Aisha, the wife of the Prophet, it is said that the Prophet's dreams would come true like the ocean's waves.[59] Just as in its predecessors, the Quran also recounts the story of Joseph and his unique ability to interpret dreams.[59]
In both Christianity and Islam dreams feature in conversion stories.[citation needed] Constantine the Great started his conversion to Christianity because while on campaign he had a dream which prophecised that he would win a battle if he adopted the Chi-Rho as his battle standard.[citation needed]
Buddhist[edit]
In Buddhism, ideas about dreams are similar to the classical and folk traditions in South Asia. The same dream is sometimes experienced by multiple people, as in the case of the Buddha-to-be, before he is leaving his home. It is described in the Mahāvastu that several of the Buddha's relatives had premonitory dreams preceding this. Some dreams are also seen to transcend time: the Buddha-to-be has certain dreams that are the same as those of previous Buddhas, the Lalitavistara states. In Buddhist literature, dreams often function as a "signpost" motif to mark certain stages in the life of the main character.[60]
Buddhist views about dreams are expressed in the Pāli Commentaries and the Milinda Pañhā.[60]
Other[edit]
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.[61] This belief and dream interpretation had been questioned since early times, such as by the philosopher Wang Chong (27–97 CE).[61]
The Babylonians and Assyrians divided dreams into "good," which were sent by the gods, and "bad," sent by demons.[62] A surviving collection of dream omens entitled Iškar Zaqīqu records various dream scenarios as well as prognostications of what will happen to the person who experiences each dream, apparently based on previous cases.[6][63] Some list different possible outcomes, based on occasions in which people experienced similar dreams with different results.[6] 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 BCE. In that century, other cultures influenced Greeks to develop the belief that souls left the sleeping body.[64] The father of modern medicine, Hippocrates (460–375 BCE), thought dreams could analyze illness and predict diseases. For instance, a dream of a dim star high in the night sky indicated problems in the head region, while low in the night sky indicated bowel issues.[65] Greek philosopher Plato (427-347) wrote that people harbor secret, repressed desires, such as incest, murder, adultery, and conquest, which build up during the day and run rampant during the night in dreams.[66] Plato's student, Aristotle (384–322 BCE), believed dreams were caused by processing incomplete physiological activity during sleep, such as eyes trying to see while the sleeper's eyelids were closed.[67] Marcus Tullius Cicero, for his part, believed that all dreams are produced by thoughts and conversations a dreamer had during the preceding days.[68] 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."[69]
The Dreaming is a common term within the animist creation narrative of indigenous Australians for a personal, or group, creation and for what may be understood as the "timeless time" of formative creation and perpetual creating.[70]
Some Indigenous American tribes and Mexican populations believe that dreams are a way of visiting and having contact with their ancestors.[71] Some Native American tribes have used vision quests as a rite of passage, fasting and praying until an anticipated guiding dream was received, to be shared with the rest of the tribe upon their return.[72][73]
Interpretation[edit]
Beginning in the late 19th century, Austrian neurologist Sigmund Freud, founder of psychoanalysis, theorized that dreams reflect the dreamer's unconscious mind and specifically that dream content is shaped by unconscious wish fulfillment. He argued that important unconscious desires often relate to early childhood memories and experiences.[7] Carl Jung and others expanded on Freud's idea that dream content reflects the dreamer's unconscious desires.
Dream interpretation can be a result of subjective ideas and experiences. One study found that most people believe that "their dreams reveal meaningful hidden truths".[74] The researchers surveyed students in the United States, South Korea, and India, and found that 74% of Indians, 65% of South Koreans and 56% of Americans believed their dream content provided them with meaningful insight into their unconscious beliefs and desires. This Freudian view of dreaming was believed significantly more than theories of dreaming that attribute dream content to memory consolidation, problem-solving, or as a byproduct of unrelated brain activity. The same study found that people attribute more importance to dream content than to similar thought content that occurs while they are awake. Americans were more likely to report that they would intentionally miss their flight if they dreamt of their plane crashing than if they thought of their plane crashing the night before flying (while awake), and that they would be as likely to miss their flight if they dreamt of their plane crashing the night before their flight as if there was an actual plane crash on the route they intended to take. Participants in the study were more likely to perceive dreams to be meaningful when the content of dreams was in accordance with their beliefs and desires while awake. They were more likely to view a positive dream about a friend to be meaningful than a positive dream about someone they disliked, for example, and were more likely to view a negative dream about a person they disliked as meaningful than a negative dream about a person they liked.
According to surveys, it is common for people to feel their dreams are predicting subsequent life events.[75] Psychologists have explained these experiences in terms of memory biases, namely a selective memory for accurate predictions and distorted memory so that dreams are retrospectively fitted onto life experiences.[75] The multi-faceted nature of dreams makes it easy to find connections between dream content and real events.[76] The term "veridical dream" has been used to indicate dreams that reveal or contain truths not yet known to the dreamer, whether future events or secrets.[77]
In one experiment, subjects were asked to write down their dreams in a diary. This prevented the selective memory effect, and the dreams no longer seemed accurate about the future.[78] Another experiment gave subjects a fake diary of a student with apparently precognitive dreams. This diary described events from the person's life, as well as some predictive dreams and some non-predictive dreams. When subjects were asked to recall the dreams they had read, they remembered more of the successful predictions than unsuccessful ones.[79]
Images and literature[edit]
Graphic artists, writers and filmmakers all have found dreams to offer a rich vein for creative expression. In the West, artists' depictions of dreams in Renaissance and Baroque art often were related to Biblical narrative. Especially preferred by visual artists were the Jacob's Ladder dream in Genesis and St. Joseph's dreams in the Gospel according to Matthew.
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Nicolas Dipre. Le songe de Jacob. c. 1500 Avignon, Petit Palais. -
José de Ribera (1591–1652). El sueño de Jacob, from Prado in Google Earth
