Dream is a form of mental activity, different from waking thought that occurs during sleep. The nature of dream activity has been characterized by many clinical and laboratory studies. These studies show that dreams are more perceptual than conceptual: things are seen and heard rather than being subjected to thought. In terms of the senses, visual experience is present in almost all dreams; auditory experience in 40 to 50 per cent; and touch, taste, smell, and pain in a relatively small percentage. A considerable amount of emotion is commonly present—usually a single, stark emotion such as fear, anger, or joy rather than the modulated emotions that occur in the waking state. Most dreams are in the form of interrupted stories, made up partly of memories, with frequent shifts of scene. This broad characterization includes a great variety of dream experiences. Many dreams collected in sleep laboratories are rather ordinary, but most people have at least some bizarre dreams. At the start of the 20th century, Sigmund Freud proposed that a mental process quite different from that used in the waking state dominates the dreaming mind. He described this “primary process” as characterized by more primitive mechanisms, by rapid shifts in energy and emotions, and by a good deal of sexual and aggressive content derived from childhood.
Research in recent years has clarified many of these aspects of dreaming, but what may be of greatest significance has been the discovery of biology of dreaming. Starting with the work of the American sleep researchers Eugene Aserinsky and Nathaniel Kleitman in 1953, studies have shown that a dream does not consist of fleeting imagery that occurs while a person awakens from sleep, but instead that it takes place during a biological state of its own. Thus, two clearly distinguishable states of sleep exist. The first state, called S-synchronized sleep, or NREM-sleep (non-rapid-eye-movement sleep), occupies most of the sleep period and is associated with a relatively low pulse and blood pressure, little activation of the autonomic nervous system, and few or no reports of dreaming. The second type of sleep, known as D-sleep (dreaming, or desynchronized, sleep), or REM-sleep (rapid-eye-movement sleep), occurs cyclically during the sleep period and is characterized by activation of the autonomic nervous system, rapid eye movements, and frequent dream reports. Typically, a person has four or five periods of D-sleep during the night, whether the dreams are remembered often, rarely, or not at all; they occur at intervals of about 90 minutes and altogether constitute about 25 per cent of the night's sleep (as much as 50 per cent in a newborn child). Evidence indicates that a dream period usually lasts from 5 to 20 minutes. Such stimuli as sounds and touches impinging on a dreamer can be incorporated into a dream if they occur during a D-period. These stimuli, however, do not initiate a D-period if one is not already in progress, so that, at least in such cases, dreams do not “protect” sleep in the way that Freud suggested. Although mental activity may be reported during NREM-sleep, these are usually short, fragmented, thought-like experiences.
Research in recent years has clarified many of these aspects of dreaming, but what may be of greatest significance has been the discovery of biology of dreaming. Starting with the work of the American sleep researchers Eugene Aserinsky and Nathaniel Kleitman in 1953, studies have shown that a dream does not consist of fleeting imagery that occurs while a person awakens from sleep, but instead that it takes place during a biological state of its own. Thus, two clearly distinguishable states of sleep exist. The first state, called S-synchronized sleep, or NREM-sleep (non-rapid-eye-movement sleep), occupies most of the sleep period and is associated with a relatively low pulse and blood pressure, little activation of the autonomic nervous system, and few or no reports of dreaming. The second type of sleep, known as D-sleep (dreaming, or desynchronized, sleep), or REM-sleep (rapid-eye-movement sleep), occurs cyclically during the sleep period and is characterized by activation of the autonomic nervous system, rapid eye movements, and frequent dream reports. Typically, a person has four or five periods of D-sleep during the night, whether the dreams are remembered often, rarely, or not at all; they occur at intervals of about 90 minutes and altogether constitute about 25 per cent of the night's sleep (as much as 50 per cent in a newborn child). Evidence indicates that a dream period usually lasts from 5 to 20 minutes. Such stimuli as sounds and touches impinging on a dreamer can be incorporated into a dream if they occur during a D-period. These stimuli, however, do not initiate a D-period if one is not already in progress, so that, at least in such cases, dreams do not “protect” sleep in the way that Freud suggested. Although mental activity may be reported during NREM-sleep, these are usually short, fragmented, thought-like experiences.
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