It is thought that for cerebral representations to be modified or updated, the cells over the whole cerebral cortex should become modifiable, and that at such times ordinary information (from the external environment) should be rigorously excluded. The time when such a condition normally exists is during sleep.
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. ...