dream dream a dream is a succession of images, ideas, emotions, and sensations that usually occur involuntarily in the mind during certain stages of sleep. humans spend about two hours dreaming per night, and each dream lasts around 5 to 20 minutes, although the dreamer may perceive the dream as being much longer than this. a dream of a girl before a sunrise c. 1830–33 by karl bryullov (1799–1852) 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 and even earlier by the ancient sumerians, figures prominently in religious texts in several traditions, and has played a lead role in psychotherapy. the scientific study of dreams is called oneirology. 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. long ago, according to writings from mesopotamia and ancient egypt, dreams dictated post-dream behaviors to an extent sharply reduced in later millennia. 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. 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, 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. 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. subjective experience and content further information: oneiromancy usha dreaming aniruddha (oleographic print) raja ravi varma (1848–1906) 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. in visitation dreams reported in ancient writings, dreamers were largely passive in their dreams, and visual content served primarily to frame authoritative auditory messaging. 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. 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. 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. 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. a soldier dreams: the trenches of wwi. jan styka (1858–1925). 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. 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. another study showed that 8% of both men's and women's dreams have sexual content. in some cases, sexual dreams may result in orgasms or nocturnal emissions. these are colloquially known as "wet dreams". 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. neurophysiology main article: cognitive neuroscience of dreams further information: neuroscience of sleep dream study is popular with scientists exploring the mind–brain problem. some "propose to reduce aspects of dream phenomenology to neurobiology." 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. non-invasive measures of brain activity like electroencephalogram (eeg) voltage averaging or cerebral blood flow cannot identify small but influential neuronal populations. also, fmri signals are too slow to explain how brains compute in real time. 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 be done on animal subjects." 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. generation the knight's dream, 1655, by antonio de pereda denied precision tools, 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. 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." 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. drawing on this knowledge, textbook author james w. kalat explains, " 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." 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." theories on function further information: rapid eye movement sleep 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. 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. 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), 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 and proposed that dreams preserve sleep by representing as fulfilled those wishes that otherwise would awaken the dreamer. freud wrote that dreams "serve the purpose of prolonging sleep instead of waking up. dreams are the guardians of sleep and not its disturbers." grandmother and granddaughter dream (1839 or 1840). taras shevchenko a turning point in theorizing about dream function came in 1953, when science published the aserinsky and kleitman paper establishing rem sleep as a distinct phase of sleep and linking dreams to rem sleep. until and even after publication of the solms 2000 paper that certified the separability of rem sleep and dream phenomena, 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...." in 2010 a harvard study was published showing experimental evidence that dreams were correlated with improved learning. 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. hartmann's 1995 proposal that dreams serve a "quasi-therapeutic" function, enabling the dreamer to process trauma in a safe place. 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. 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. 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. religious and other cultural contexts 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," 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.... hindu 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. 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