dream, mental activity associated with the rapid-eye-movement (REM) period of sleep. It is commonly made up of a number of visual images, scenes or thoughts expressed in terms of seeing rather than in those of the other senses or in words. Electroencephalograph studies, measuring the electrical activity of the brain during REM sleep, have shown that young adults dream for 11/2 to 2 hours of every 8-hour period of sleep. Infants spend an average of 50% of their sleep in the REM phase (they are believed to dream more often than adults) a figure which decreases steadily with age. During dreams, blood pressure and heart rate increase, and breathing is quickened, but the body is otherwise immobile. Studies have shown that sleepers deprived of dream-sleep are likely to become irritable and lose coordination skills. Unusually frightening dreams are called nightmares, and daydreams are constructed fantasies that occur while the individual is awake. Studies have demonstrated the existence of lucid dreaming, where the individual is aware that he is dreaming and has a degree of control over his dream.

Sigmund Freud, in his pioneering work The Interpretation of Dreams (1900, tr. 1913), was one of the first to emphasize dreams as keys to the unconscious. He distinguished the manifest content of dreams—the dream as it is recalled by the individual—from the latent content or the meaning of the dream, which Freud saw in terms of wish fulfillment. C. G. Jung held that dreams function to reveal the unconscious mind, anticipate future events, and give expression to neglected areas of the dreamer's personality. Another theory, which PET scan studies appear to support, suggests that dreams are a result of electrical energy that stimulates memories located in various regions of the brain.

Series of thoughts, images, or emotions occurring during sleep, particularly sleep accompanied by rapid eye movement (REM sleep). Dream reports range from the very ordinary and realistic to the fantastic and surreal. Humans have always attached great importance to dreams, which have been variously viewed as windows to the sacred, the past and the future, or the world of the dead. Dreams have provided creative solutions to intellectual and emotional problems and have offered ideas for artistic pursuits. A type of cognitive synthesis that facilitates conscious insight may occur subconsciously during dreaming. The most famous theory of the significance of dreams is the psychoanalytic model of Sigmund Freud; in Freud's view, desires that are ordinarily repressed (hidden from consciousness) because they represent forbidden impulses are given expression in dreams, though often in disguised (i.e., symbolic) form.

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