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 [Image]  [Image]                                                   [Image]
                   Stages of Sleep and Dreaming

 [Image]  Dreaming [sleeping person] The physiology of dreaming     [Image]
            occurs                   was first understood in 1953,
            solely when researchers characterized the human sleep
            during cycle. They found that sleep in humans is
               REM initiated by the hypnogogic state, a period of
           sleep.  several minutes when thoughts consist of
                   fragmented images or minidramas. The hypnogogic
                   state is followed by slow-wave sleep, so called
                   because at that time the brain waves of the
                   neocortex (the convoluted outer mantle of the
                   brain) are low in frequency and large in
                   amplitude. These signals are measured as
                   electroencephalographic (EEG) recordings.

                   Researchers also discovered that a night's
                   sleep is punctuated by periods in which the EEG
                   readings are irregular in frequency and low in
                   amplitudeãsimilar to those observed in awake
                   individuals. These periods of mental activity
                   are called REM sleep. Dreaming takes place
                   solely during these periods. While in REM
                   sleep, motor neurons are inhibited, preventing
                   the body from moving freely but allowing
                   extremities to remain slightly active. Eyes
                   move rapidly in unison under closed lids,
                   breathing becomes irregular and heart rate
                   increases.

                   The first REM stage of the night follows 90
                   minutes of slow-wave sleep and lasts for 10
                   minutes. The second and third REM periods
                   follow shorter slow-wave sleep episodes but
                   grow progressively longer themselves. The
                   fourth and final REM interval lasts 20 to 30
                   minutes and is followed by awakening. If a
                   dream is remembered at all, it is most often
                   the one that occurred in this last phase of REM
                   sleep.

                   This sleep cycleãalternating slow-wave and REM
                   sleepãappears to be present in all placental
                   and marsupial mammals. Mammals exhibit the
                   various REM-associated characteristics observed
                   in humans, including EEG readings similar to
                   those of the awake state. Animals also dream.
                   By destroying neurons in the brain stem that
                   inhibit movement during sleep, researchers
                   found that sleeping cats rose up and attacked
                   or were startled by invisible
                   objectsãostensibly images from dreams.

                   By studying subprimate animals, scientists have
                   discovered additional neurophysiological
                   aspects of REM sleep. They determined that
                   neural control of this stage of the sleep cycle
                   is centered in the brain stem (the brain region
                   closest to the spinal cord) and that during REM
                   sleep neural signalsãcalled
                   pontine-geniculate-occipital (PGO) cortex
                   spikesãproceed from the brain stem to the
                   center of visual processing, the visual cortex.
                   Brain stem neurons also initiate a sinusoidal
                   wave (one resembling a sine curve) in the
                   hippocampus. This brain signal is called theta
                   rhythm.

                   At least one animal experiences slow-wave but
                   not REM sleepãand consequently, does not
                   exhibit theta rhythm when asleep. This animal
                   is the echidna, or spiny anteater, an
                   egg-laying mammal (called a monotreme) that
                   provides some insight into the origin of
                   dreaming. The absence of REM sleep in the
                   echidna suggests that this stage of the sleep
                   cycle evolved some 140 million years ago, when
                   marsupials and placentals diverged from the
                   monotreme line. (Monotremes were the first
                   mammals to develop from reptiles.)

                   By all evolutionary criteria, the perpetuation
                   of a complex brain process such as REM sleep
                   indicates that it serves an important function
                   for the survival of mammalian species.
                   Understanding that function might reveal the
                   meaning of dreams.

                   [Image] Freud Reconsidered

[Copyright 1999, Peregrine Publishers, Inc. All Rights Reserved.]