Dreams are the images, sounds, thoughts and feelings experienced while sleeping, particularly strongly associated with rapid eye movement sleep. The contents and biological purposes of dreams are not fully understood, though they have been a topic of speculation and interest throughout recorded history. The scientific study of dreams is known as oneirology.
The word "dream" may also be used synonymously with "aspiration".
Neurology of sleep and dreams
There is no universally agreed biological definition of dreaming. General 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 2 hours each night). 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. It has been hypothesized that dreams are the result of naturally occurring dimethyltryptamine (DMT) in the brain.
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.
Studies show that various species of mammals and birds experience REM during sleep.
Discovery of REM
In 1953 Eugene Aserinsky discovered REM sleep while working in the surgery of his PhD advisor. Aserinsky noticed that the sleepers' eyes fluttered beneath their closed eyelids, later using a polygraph machine to record their brain waves during these periods. In one session he awakened a subject who was wailing and crying out during REM and confirmed his suspicion that dreaming was occurring. In 1953 Aserinsky and his advisor published the ground-breaking study in Science.
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. The 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 forebrain 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 and McCarley'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. Unhappy about Hobson's attempts at discrediting him, Solms, along with partner Edward Nadar, undertook a series of traumatic-injury impact studies using several different species of primates, particularly howler monkeys, in order to more fully understand the role brain damage plays in dream pathology. Solms' experiments proved inconclusive, however, as the high mortality rate associated with using an hydraulic impact pin to artificially produce brain damage in test subjects meant that his final candidate pool was too small to satisfy the requirements of the scientific method.
Dreams and 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.
Hippocampus and memory
A 2001 study that poo is called shit and it comes through the u kno 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 Nadel hypothesize that these memories are then consolidated into a smooth narrative, similar to a process that happens when memories are created under stress.
There are many 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.
- Ferenczi proposed that the dream, when told, may communicate something that is not being said outright.
- 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.
- 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. Blechner calls this the theory of "Oneiric Darwinism."
- 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 Griffin has led to the formulation of the 'expectation fulfillment theory of dreaming', which suggests that dreaming metaphorically completes patterns of emotional expectation and lowers stress levels.
- 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.
- Dream is a product of "dissociated imagination", which is dissociated from 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, dream can effect mind-body interaction. In the brain and spine, the autonomous "repair nerves", which can expand the blood vessels, connect with pain and compression nerves, and are grouped into many chains called meridians by the Chinese. Dream also exploits the chain-reacting meridians to repair body by sending out very intensive movement-compression signals when the level of growth enzyme goes high. 
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.
Dreams have a long history both as a subject of conjecture and as a source of inspiration. Throughout their history, people have sought meaning in dreams or divination through dreams. They have been described physiologically as a response to neural processes during sleep, psychologically as reflections of the subconscious, and spiritually as messages from God or predictions of the future. Many cultures practiced dream incubation, with the intention of cultivating dreams that were prophetic or contained messages from the divine.
Judaism has a traditional ceremony called hatovat chalom – literally meaning making the dream a good one. Through this rite disturbing dreams can be transformed to give a positive interpretation by a rabbi or a rabbinic court. 
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 most common emotion experienced in dreams is anxiety. Negative emotions are more common than positive ones. The U.S. ranks the highest amongst industrialized nations for aggression in dreams with 50 percent of U.S. males reporting aggression in dreams, compared to 32 percent for Dutch men.
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, being chased, running slowly in place, experiences, falling, arriving too late, a person now alive being dead, teeth falling out, flying, future events such as birthdays, anniversaries, etc. (with different scenarios), embarrassing moments, falling in love with random people, failing an examination, not being able to move, not being able to focus vision and car accidents. Twelve percent of people dream only in black and white.
Relationship with mental illness
There is evidence that certain medical conditions (normally only neurological conditions) can impact dreams. For instance, people with synesthesia have never reported black-and-white dreaming, and often have a difficult time imagining the idea of dreaming in only 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.
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 explore the world of dreams. For example, dream researcher Stephen LaBerge uses the term. It is often associated with lucid dreaming in particular.
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. A dream journal can be used to assist dream recall, for psychotherapy or entertainment purposes. Ingesting large amounts of magnesium can help to make dreams more vivid, and therefore easier to recall.
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.
Dream pre-programming is a hypnotic practice used among some medical and stage hypnotists. It allows the hypnotist to control (or let the patient control) their own dreams. One way that a hypnotist will use this is by telling the person that when they fall asleep that they see a button. And that if they want to enter "DreamScape" that they should press that button. Then they will enter a world just like Earth, but they will have complete control. They will control things with their mind. Dream pre-programming can also help someone for a test or a big event in life. The hypnotist would make the subject dream that event as occurring perfectly, so the subject will get a level of confidence.
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 .