What is dream?

Dreams are the images, thoughts and feelings experienced while asleep, particularly strongly associated with rapid eye movement sleep. The contents and purpose of dreams are not fully understood, though they have been a topic of speculation and interest throughout recorded history.

Stages of sleep
When the body decides that it is time to sleep, neurons near the eyes begin to send signals throughout the body. According to Michael Smith, these neurons are located in such close proximity to neurons that control eyelid big muscles that the eyelids begin to grow heavy.Glands begin to secrete a hormone that helps induce sleep and neurons send signals to the spinal cord which cause the body to relax.

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

Functions of dreams
There are many hypotheses about the function of dreams. 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 may also 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.Dreams also let the mind express things that would normally be suppressed in the waking world, thus keeping itself in harmony. Dreams may also offer a view at how future events might proceed; this is similar to running future events through the mind, for instance: a work presentation, a job interview, or a first date.

Jung suggested that dreams may compensate for one-sided attitudes held in waking consciousness.Ferencziproposed that the dream, when told, may communicate something that is not being said outright. There have also been analogies made with the cleaning-up operations of computers when they are off-line. Dreams may remove parasitic nodes and other "junk" from the mind during sleep.Dreams may also 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. Blechnercalls this the theory of "Oneiric Darwinism." Dreams may also regulate mood.Hartmannsays 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 fulfilment theory of dreaming', which suggests that dreaming metaphorically completes patterns of emotional expectation and lowers stress levels

Recurring 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.

Common themes
Content-analysis studies have identified common reported themes in dreams. These include: situations relating to school, being chased, running slowly/in place,falling, arriving too late, a person now alive being dead, teeth falling out, flying, embarrassing moments, failing an examination, or a car accident. Twelve percent of people dream only in black and white.


Disease-associated differences
There is evidence that certain diseases (normally only neurological diseases) 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 black and white only.