It is thought that mental activity takes place during D/REM and S/NREM sleep, but that the mental activity during D/REM sleep is the most dream-like, while that occurring during S/NREM sleep is much more awake-like. Consequently, it is the D/REM sleep phase that is thought to be correspondent with the actual experience of dreaming, while the subsequent S/NREM phase is correspondent with mental activity related to the experience of the preceding dream phase. (Marjorie L. Rand, 2002)
Normally, dream sleep appears at 70-90 minute intervals throughout the sleep period in adults. There is, however, evidence that the physiological and neurophysiological activity normally correspondent with dream sleep also occurs with the same periodicity, but less intensely or evidently, throughout the awake state.
Underlying the two psychological systems considered to interact during the process of dreaming may be two physiological or neuroanatomical systems. Dream sleep may involve the preponderant or emergent activity of a neuroanatomical region different from the region that normally is preponderantly active in the awake state. (Cheryl H. Alexander, 2000).
One factor thought to contribute to the onset of sleep, particularly dream sleep, is a reduction of externally generated stimuli that reach the higher and conscious level of the brain. As a result of this reduction there may be facilitated a release of internally generated past impressions or memories from within the lower neuroanatomical and unconscious levels. Within the higher, or cerebral, level of the brain, the emergence of information released from the lower level may also involve a decreased activity of processes, or switching off of the system that would normally in the awake state inhibit such preponderant emergence. (Edmund T. Rolls, 2000).
Relative to the S/NREM sleep phase, the D/REM sleep phase is accompanied by increased cerebral blood flow; increased activity of the autonomic nervous system; increased "spontaneous" activity of the reticular formation; a higher response threshold in the midbrain reticular formation to auditory as well as direct stimulation; and a higher threshold for behavioral awakening. (Simon J. Sherwood, 2002).
Among the phenomena evident during D/REM sleep are cerebral desynchronization; hippocampal theta waves of 5-6 Hertz, a depression of spontaneous and reflex activity in the muscles and a general atonia, especially of the antigravity muscles. The preceding phenomena constitute a tonic background on which are superimposed the following phasic phenomena: large amplitude waves occurring in the pontine reticular formation, lateral geniculate nuclei and occipital cortex, which are referred to as PGO activity and which normally occur coincidentally with REM bursts; short-term increases in the frequency of the hippocampal theta rhythm; increased muscle activity and myoclonic twitches; and short fluctuations within the autonomic nervous system. Also involved in D/REM sleep are the parietal lobe, the temporal lobe, the cerebral limbic regions, and the pulvinar of the thalamus. (Elena Miro, M. Carmen Cano, Lourdes Espinosa-Fernandez, Gualberto Buela-Casal, 2003).
Both the tonic and phasic phenomena of dream sleep may be regarded basically as an ascending excitatory component