Consciousness, instead of being essential to living bodies, is never even persistent in any living body. It is invariably an intermittent or periodic phenomenon and any tendency toward persistence is generally, if not always, detrimental or destructive to the organism. The average period of consciousness of living animals is about twelve hours, followed normally by an equal period of oblivion, that is, a period in which there is no consciousness or cognition.
These periods correspond regularly to a semi-rotational period of the earth. The period of oblivion, or sleep, corresponds in the case of most animals with the semi-rotational period of darkness and low temperature. The probable reason for this correspondence is the fact that during the period of darkness the avenues of visual sensation would be comparatively useless, if that were the period of activity. The operation of the organic law of the survival of those best suited to their environment, has produced races and types of organisms whose requirements in periods of activity and oblivion correspond with earthly conditions.
A dream is a phenomenon of the same order as consciousness. It is a kind of consciousness. It is produced under conditions similar to, but not identical with, those which produce ordinary or normal consciousness. In the dream there is cognition, brain activity and the dissipation of nerve energy. There is re-cognition and memory. As there is brain activity, there must be alteration of the brain tissue. There is, therefore, a recording of the phenomenon or dream itself similar to the record of all other conscious states.
Consequently, a dream is remembered as a past experience. The brain activity of a dream, however, is different from the normal brain activity, and the dream is a conscious state excited and maintained while the avenues of sensation are inactive or only partly active. The normal or ordinary conscious state is produced by brain activity while the avenues of sensation are open and active. In the dream the stimulus of brain activity must be entirely or chiefly from within the brain, because there is no normal, voluntary communication through the organs of sense with outer existence.
Such a conscious state must be chiefly the product of past and recorded experiences only, or of past experiences juxtaposed upon the imperfect cognitions produced through partly active sensory tracts; but not necessarily or likely of complete or chronologically arranged or relatively connected experiences. They may be fragmentary and jumbled together haphazard, or may follow an orderly chronological sequence. depending upon the cause and nature of the stimulus of brain activity, and also to a great extent upon the condition and cerebral idiosyncrasies of the individual. These abnormal brain activities produce cognitions which are merely the haphazard combinations of the most easily available brain records, and the recollection of these cognitions afterward appears grotesque to the normal consciousness.
***Excerpt from Charles John Reed: The Law of Vital Infusion and the Phenomenon of Consciousness