Biology of Dreaming

Research in recent years has clarified many of these aspects of dreaming, but what may be of greatest significance has been the discovery of a biology of dreaming (see Sleep). Starting with the work of 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 a dream 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 percent of the night’s sleep (as much as 50 percent 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, thoughtlike experiences.