Dream, a series of sensations, images, or thoughts that pass through a sleeping person's mind. An unpleasant or terrifying dream is called a nightmare. It is probable that everyone dreams. A person who denies having dreams has possibly failed to recall them. One who is blind from birth dreams of sound, touch, and ideas.

Dreaming, like all thought, occurs in the part of the brain called the cerebral cortex. The cortex is stimulated by chemicals pro-duced in a section of the brain stem called the pons. Dream activity is believed to center in an area of the cortex that controls eye movements, but no other body activity. For this reason, there is considerable movement of the eyes behind the closed lids, but very little body movement, during a dream—even during a dream highlighted by violent physical exertion.

Most persons dream four or five times a night. A dream may be in black and white or in color. Usually it lasts from about 10 to about 40 minutes, the length increasing as morning approaches. Dreams generally occur during that stage of light sleep called REM. (REM refers to the rapid eye movement that accompanies dreaming.)

Why people dream—that is, what purpose dreams serve—is not definitely known. Experiments in sleep and dreaming have shown, however, that there is a psychologcal and physiological need to dream. When persons in sleep laboratories were deprived of their REM sleep for one or more nights, they dreamed about 60 per cent more if lef to sleep undisturbed on subsequent nights. They seemed to be trying to make up for lost dreams. Many individuals deprived of REM sleep showed psychological and physical abnormalities during their waking hours; others, however, showed no obvious distur-bances. In general, however, experts agree that dreaming is essential to mental and physical well-being.

Dreams have aroused much wonder and fear since primitive times, and various superstitions have grown up around them. Some ancient peoples thought dreams were messages from their gods.

The first systematic study of dreams was begun in the late 1800's by Dr. Sigmund Freud, father of psychoanalysis. In The Interpretation of Dreams (1900) Freud claimed that certain desires are checked by a censoring process acting in the conscious mind. In sleep, this censorship does not function. Impulses pass freely from unconscious to conscious mind, where these impulses are revealed in the form of dreams. According to Freud, dreams gratify those desires that a person would never express while he is awake, and dreams, therefore, give important clues to emotional disturbances.

Psychiatrists hold a variety of opinions on the value of dream analysis. Some feel dream analysis is of little value; others put even more emphasis on it than did Freud. In contrast to Freud, however, many modern psychiatrists tend to view dreams as attempts to solve problems rather than as the fulfillment of unconscious desires. Some researchers reject both these views; they say dreams are a means of sorting out for rejection or storage in the memory the information gathered during wakefulness.