Accumulated observation shows that dreams are strongly associated with rapid eye movement (REM) sleep, during which an electroencephalogram shows brain activity to be most like wakefulness. Participant-nonremembered dreams during non-REM sleep are normally more mundane in comparison. During a typical lifespan, a human spends a total of about six years dreaming (which is about two hours each night). Most dreams last only 5 to 20 minutes. It is unknown where in the brain dreams originate, if there is a single origin for dreams or if multiple portions of the brain are involved, or what the purpose of dreaming is for the body or mind.
During REM sleep, the release of certain neurotransmitters is completely suppressed. As a result, motor neurons are not stimulated, a condition known as REM atonia. This prevents dreams from resulting in dangerous movements of the body.
Animals have complex dreams and are able to retain and recall long sequences of events while they are asleep. Studies show that various species of mammals and birds experience REM during sleep, and follow the same series of sleeping states as humans.
Despite their power to bewilder, frighten us or amuse us, dreams are often ignored in mainstream models of cognitive psychology. As methods of introspection were replaced with more self-consciously objective methods in the social sciences in 1930s and 1940s, dream studies dropped out of the scientific literature. Dreams were neither directly observable by an experimenter nor were subjects’ dream reports reliable, being prey to the familiar problems of distortion due to delayed recall, if they were recalled at all. More often dreams are, of course, forgotten entirely, perhaps due to their (according to Freud) prohibited character. Altogether these problems seemed to put them beyond the realm of science.
The discovery that dreams take place primarily during a distinctive electrophysiological state of sleep, Rapid Eye Movement (REM) sleep, which can be identified by objective criteria, led to rebirth of interest in this phenomenon. When REM sleep episodes were timed for their duration and subjects woken to make reports before major editing or forgetting could take place, it was determined that subjects accurately matched the length of time they judged the sleep the dream narrative to be ongoing to the length of REM sleep that preceded the awakening. This close correlation of REM sleep and dream experience was the basis of first series of reports describing the nature of dreaming: that it is regular nightly, rather than occasional, phenomenon, and a high-frequency activity within each sleep period occurring at predictable intervals of approximately every 60–90 minutes in all humans throughout the life span. REM sleep episodes and the dreams that accompany them lengthen progressively across the night, with the first episode being shortest, of approximately 10–12 minutes duration, and the second and third episodes increasing to 15–20 minutes. Dreams at the end of the night may last as long as 15 minutes, although these may be experienced as several distinct stories due to momentary arousals interrupting sleep as the night ends. Dream reports can be reported from normal subjects on 50% of the occasion when an awakening is made prior to the end of the first REM period. This rate of retrieval is increased to about 99% when awakenings are made from the last REM period of the night. This increase in the ability to recall appears to be related to intensification across the night in the vividness of dream imagery, colors and emotions. The dream story itself in the last REM period is farthest from reality, containing more bizarre elements, and it is these properties, coupled with the increased likelihood of spontaneous arousals allowing waking review to take place, that heighten the chance of recall of the last dream.
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Activation synthesis theory
In 1976 J. Allan Hobson and Robert McCarly proposed a new theory that changed dream research, challenging the previously held Freudian view of dreams as subconscious wishes to be interpreted. Activation synthesis theory asserts that the sensory experiences are fabricated by the cortex as a means of interpreting chaotic signals from the pons. They propose that in REM sleep, the ascending cholinergic PGO (ponto-geniculo-occipital) waves stimulate higher midbrain and forebrain cortical structures, producing rapid eye movements. The activated fore brain then synthesizes the dream out of this internally generated information. 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 originated in the brain stem during REM sleep. However, research by Mark Solms suggests that dreams are generated in the forebrain, and that REM sleep and dreaming are not directly related. 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 brain stem damage. This observation forced him to question Hobson's prevailing theory which marked the brain stem as the source of the signals interpreted as dreams. Solms viewed the idea of dreaming as a function of many complex brain structures as validating Freudian dream theory, an idea that drew criticism from Hobson. In 1978, Solms, along with partners William Kauffman and Edward Nadar, undertook a series of traumatic-injury impact studies using several different species of primates, particularly howler monkeys, in order to disprove Hobson's postulation that the brain stem played a significant role in dream pathology. Unfortunately, Solms' experiments proved inconclusive, as the high mortality rate associated with using an hydraulic impact pin to artificially induce brain damage in test subjects meant that his final candidate pool was too small to satisfy the requirements of the scientific method. 
Combining Hobson's activation synthesis hypothesis with Solms's 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 to the long-term memory, though there is not much evidence backing up this so-called "consolidation." NREM sleep processes the conscious-related memory (declarative memory), and REM sleep processes the unconscious related memory (procedural memory).
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 will descend to a very low level as the inputs from the sensory are basically disconnected. This will trigger 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).
Dreams 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.
Dreams for linking and consolidation of semantic memories
A 2001 study showed evidence that illogical locations, characters, and dream flow may help the brain strengthen the linking and consolidation of semantic memories. These conditions may occur because, during REM sleep, the flow of information between the hippocampus and neocortex is reduced. Increasing levels of the stress hormone Cortisol late in sleep (often during REM sleep) cause this decreased communication. One stage of memory consolidation is the linking of distant but related memories. Payne and Nadal hypothesize that these memories are then consolidated into a smooth narrative, similar to a process that happens when memories are created under stress.
Dreams for removing junk or handling information
Hughlings Jackson (1932) viewed that sleep serves to sweep away unnecessary memories and connections from the day. This was recently revised by Crick and Mitchison (1983) and stated that dreams are like the cleaning-up operations of computers when they are off-line, removing parasitic nodes and other "junk" from the mind during sleep. However, the opposite view that dreaming has an information handling, memory-consolidating function (Hennevin and Leconte, 1971) is also common. Dreams are a result of the spontaneous firings of neural patterns while the brain is undergoing memory consolidation during sleep.
Dreams for adaptive evolutionary purposes
Several dream psychologists have taken a Darwinian approach to explaining dreaming. Harvard psychologist Deirdre Barrett describes dreaming as simply “thinking in different biochemical state” and believes we continue to work on all the same problems—personal and objective—in that state. Her research finds that anything—math, music, business decisions—may get aided by dreams, but the two areas most likely to reach solutions in this state are 1) anything where visualization contributes to the solution, whether in art or invention of technological devices and 2) any problem where the solution lies in “thinking outside the box”—i.e. where the person is stuck because the conventional wisdom on how to approach the problem is wrong. Blechner suggests that dreams create 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. He calls this the theory of "Oneiric Darwinism."
Dreams for testing and selecting mental schemas
Coutts hypothesizes that dreams modify and test mental schemas during sleep during a process he calls emotional selection, and that only schema modifications that appear emotionally adaptive during dream tests are selected for retention, while those that appear maladaptive are abandoned or further modified and tested.
Dreams as the result of DMT in the brain
Dreams are the result of dimethyltryptamine (DMT) in the brain. A biochemical mechanism for this was proposed by the medical researcher J. C. Callaway, who suggested in 1988 that DMT might be connected with visual dream phenomena, where brain DMT levels are periodically elevated to induce visual dreaming and possibly other natural states of mind.
Dreams are a product of "dissociated imagination", which is dissociated from the conscious self and draws material from sensory memory for simulation, with sensory feedback resulting in hallucination. By simulating the sensory signals to drive the autonomous nerves, dreams can affect mind-body interaction. In the brain and spine, the autonomous "repair nerves", which can expand the blood vessels, connect with pain and compression nerves. These nerves are grouped into many chains called meridians in Chinese medicine. While dreaming, the body also employs the chain-reacting meridians to repair the body and help it grow and develop by sending out very intensive movement-compression signals when the level of growth enzymes increase. 
Other Hypotheses on dreaming
There are many other hypotheses about the function of dreams, including:
- During the night there may be many external stimuli bombarding the senses, but the mind interprets the stimulus and makes it a part of a dream in order to ensure continued sleep. The mind will, however, awaken an individual if they are in danger or if trained to respond to certain sounds, such as a baby crying.
- Dreams allow the repressed parts of the mind to be satisfied through fantasy while keeping the conscious mind from thoughts that would suddenly cause one to awaken from shock.
- Freud suggested that bad dreams let the brain learn to gain control over emotions resulting from distressing experiences.
- Jung suggested that dreams may compensate for one-sided attitudes held in waking consciousness.
- 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.
- Ferenczi proposed that the dream, when told, may communicate something that is not being said outright.
- Dreams regulate mood.
- Hartmann says dreams may function like psychotherapy, by "making connections in a safe place" and allowing the dreamer to integrate thoughts that may be dissociated during waking life.
- More recent research by psychologist Joe Griffin, following a twelve year review of data from all major sleep laboratories, led to the formulation of the expectation fulfilment theory of dreaming, which suggests that dreaming metaphorically completes patterns of emotional expectation in the autonomic nervous system and lowers stress levels in mammals.
From the 1940s to 1985, Calvin S. Hall collected more than 50,000 dream reports at Western Reserve University. In 1966 Hall and 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. It was found that people all over the world dream of mostly the same things. Hall's complete dream reports became publicly available in the mid-1990s by Hall's protégé William Domhoff, allowing further different analysis.
Personal experiences from the last day or week are frequently incorporated into dreams.
The Hall data analysis shows that sexual dreams occur no more than 10 percent of the time and are more prevalent in young to mid teens. Another study showed that 8% of men's and women's dreams have sexual content. In some cases, sexual dreams may result in orgasm or nocturnal emission. These are commonly known as wet dreams.
While the content of most dreams is dreamt only once, many people experience recurring dreams—that is, the same dream narrative is experienced over different occasions of sleep. Up to 70% of females and 65% of males report recurrent dreams.
Content-analysis studies have identified common reported themes in dreams. These include: situations relating to school (adolescents), being chased or attacked, running slowly in place, falling, arriving too late, a person alive in reality dead in the dream , a person who is dead in real life alive in the dream, teeth falling out, flying, future events such as birthdays, anniversaries, etc. (with different scenarios), Past events in your life (with different scenarios) embarrassing moments, falling in love with random people, failing an examination, not being able to move or focus vision, car accidents, being accused of a crime you didn't commit, suddenly finding yourself naked, going to the toilet, losing your car, not knowing where you are and many more.
Colour vs. black and white
Dreams were historically used for healing (as in the asclepieions found in the ancient Greek temples of Asclepius) as well as for guidance or divine inspiration. Some Native American tribes 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.
During the late 19th and early 20th centuries, both Sigmund Freud and Carl Jung identified dreams as an interaction between the unconscious and the conscious. They also assert together that the unconscious is the dominant force of the dream, and in dreams it conveys its own mental activity to the perceptive faculty. While Freud felt that there was an active censorship against the unconscious even during sleep, Jung argued that the dream's bizarre quality is an efficient language, comparable to poetry and uniquely capable of revealing the underlying meaning.
Fritz Perls presented his theory of dreams as part of the holistic nature of Gestalt therapy. Dreams are seen as projections of parts of the self that have been ignored, rejected, or suppressed. Jung argued that one could consider every person in the dream to represent an aspect of the dreamer, which he called the subjective approach to dreams. Perls expanded this point of view to say that even inanimate objects in the dream may represent aspects of the dreamer. The dreamer may therefore be asked to imagine being an object in the dream and to describe it, in order to bring into awareness the characteristics of the object that correspond with the dreamer's personality.
Relationship with medical conditions
There is evidence that certain medical conditions (normally only neurological conditions) can impact dreams. For instance, people with synesthesia have never reported entirely black-and-white dreaming, and often have a difficult time imagining the idea of dreaming in only black and white.
Dreams and psychosis
A number of thinkers have commented on the similarities between the phenomenology of dreams and that of psychosis. Features common to the two states include thought disorder, flattened or inappropriate affect (emotion), and hallucination. Among philosophers, Kant, for example, wrote that ‘the lunatic is a wakeful dreamer’. Schopenhauer said: ‘A dream is a short-lasting psychosis, and a psychosis is a long-lasting dream.’ In the field of psychoanalysis, Freud wrote: ‘A dream then, is a psychosis’, and Jung: ‘Let the dreamer walk about and act like one awakened and we have the clinical picture of dementia praecox.’
McCreery has sought to explain these similarities by reference to the fact, documented by Oswald, that sleep can supervene as a reaction to extreme stress and hyper-arousal. McCreery adduces evidence that psychotics are people with a tendency to hyper-arousal, and suggests that this renders them prone to what Oswald calls ‘microsleeps’ during waking life. He points in particular to the paradoxical finding of Stevens and Darbyshire that patients suffering from catatonia can be roused from their seeming stupor by the administration of sedatives rather than stimulants.
Other associated phenomena
Lucid dreaming is the conscious perception of one's state while dreaming. In this state a person usually has control over characters and the environment of the dream as well as the dreamer's own actions within the dream. The occurrence of lucid dreaming has been scientifically verified.
Oneironaut is a term sometimes used for those who lucidly dream.
Dreams of absent-minded transgression
Dreams of absent-minded transgression (DAMT) are dreams wherein the dreamer absentmindedly performs an action that he or she has been trying to stop (one classic example is of a quitting smoker having dreams of lighting a cigarette). Subjects who have had DAMT have reported waking with intense feelings of guilt. One study found a positive association between having these dreams and successfully stopping the behavior.
Dreaming and the "real world"
Dreams can link to actual sensations, such as the incorporation of environmental sounds into dreams such as hearing a phone ringing in a dream while it is ringing in reality, or dreaming of urination while wetting the bed. Except in the case of lucid dreaming, people dream without being aware that they are doing so. Some philosophers have concluded that what we think as the "real world" could be or is an illusion (an idea known as the skeptical hypothesis about ontology). The first recorded mention of the idea was by Zhuangzi, and was also discussed in Hinduism; Buddhism makes extensive use of the argument in its writings. It was formally introduced to western philosophy by Descartes in the 17th century in his Meditations on First Philosophy.
The recall of dreams is extremely unreliable, though it is a skill that can be trained. Dreams can usually be recalled if a person is awakened while dreaming. Women tend to have more frequent dream recall than men. Dreams that are difficult to recall may be characterized by relatively little affect, and factors such as salience, arousal, and interference play a role in dream recall. Often, a dream may be recalled upon viewing or hearing a random trigger or stimulus. A dream journal can be used to assist dream recall, for psychotherapy or entertainment purposes. For some people, vague images or sensations from the previous night's dreams are sometimes spontaneously experienced in falling asleep. However they are usually too slight and fleeting to allow dream recall.778777
One theory of déjà vu attributes the feeling of having previously seen or experienced something to having dreamt about a similar situation or place, and forgetting about it until one seems to be mysteriously reminded of the situation or place while awake. Déjà vu comes from the French language, meaning "Already seen."
In one use of the term, "dream incorporation" is a phenomenon whereby an external stimulus, usually an auditory one, becomes a part of a dream, eventually then awakening the dreamer. There is a famous painting by Salvador Dalí that depicts this concept, titled "Dream Caused by the Flight of a Bee around a Pomegranate a Second Before Awakening" (1944).
The term "dream incorporation" is also used in research examining the degree to which preceding daytime events become elements of dreams. Recent studies suggest that events in the day immediately preceding, and those about a week before, have the most influence.
Modern popular culture often conceives of dreams, like Freud, as expressions of the dreamer's deepest fears and desires. In films such as Spellbound (1945) or The Manchurian Candidate (1962), the protagonists must extract vital clues from surreal dreams.
Most dreams in popular culture are, however, not symbolic, but straightforward and realistic depictions of their dreamer's fears and desires. Dream scenes may be indistinguishable from those set in the dreamer's real world, a narrative device that undermines the dreamer's and the audience's sense of security and allows horror movie protagonists, such as those of Carrie (1976), Friday the 13th (1980) or An American Werewolf in London (1981) to be suddenly attacked by dark forces while resting in seemingly safe places.
In speculative fiction, the line between dreams and reality may be blurred even more in the service of the story. Dreams may be psychically invaded or manipulated (the Nightmare on Elm Street films, 1984–1991) or even come literally true (as in The Lathe of Heaven, 1971). Such stories play to audiences’ experiences with their own dreams, which feel as real to them.[