In the vast majority of homeotherms, birds and mammals, it has become classic to oppose two sleep states that are characterized by different behavioral, electrophysiological and energetic aspects: slow wave sleep and sleep with rapid cortical activity, also called Paradoxical Sleep or Rapid Eye Movement (REM Sleep). The relationship of these two states with the Mind must therefore be studied successively.
Slow wave sleep
It is characterized by the disappearance of two major conditions that accompany conscious awakening.
1) On the one hand, cortical activity is not faster. It slows down and is invaded by an automatic activity called “spindles”, of thalamic origin. It is accepted that thalamo-cortical circuits then prevent any possibility of conscious integration or perceptivity. As the depth of sleep increases, high voltage slow waves appear which are initiated from the cortex according to unknown mechanisms.
2) Sleep is also accompanied by a marked decrease in glucose and oxygen consumption by the cerebral cortex, while energy reserves are then stored in the glial cells in the form of glycogen.
The mind during slow wave sleep: a first remark is necessary. It had already been well expressed by the Cambridge theologian Ralph Cudworth in 1678, in response to René Descartes: “that there may exist, however, a vital energy without clear consciousness and without explicit attention or self-perception seems plausible. First of all, the philosophers who have made the essence of the soul consist in reflection, and the essence of reflection in clear and explicit consciousness, can in no way make us believe that the human soul in deep sleep, lethargy or apoplexy… can ever remain for a single instant without explicitly conscious reflection. If it were so, by the principles of their philosophy they should ipso facto cease to be […] It is certain that our soul itself is not always conscious of what goes into it. For the sleeping geometrist does not cease for all that to have in him in a certain way all his geometrical theorems, in the same way the sleeping musician does not preserve any less all his musical aptitudes and his melodies…”.
We have known for a long time that memory can resist deep sleep (and that it can even persist in animals in the absence of any cerebral electrical activity in hypothermia).
What about consciousness, perception or insight into the external environment? It has been proven that learning ceases completely during sleep and “hypnopaedia” is an illusion, although sleep after learning can have a beneficial influence. In fact, the data concerning the current of consciousness during sleep comes to us mainly from studies carried out on humans, awakened during sleep under electroencephalographic control. These experiments have been carried out for 30 years, in many laboratories. It is quite clearly established, however, that a subject suddenly awakened during the sleep that precedes the first dream of the night is unable to remember the slightest thought, and often cannot even guess the duration of his sleep, which seems to translate into a suspension of all consciousness, even the consciousness of being asleep (the snorer does not know that he snores). In 30% of cases, however, it is possible to obtain the memory of an abstract type of thought totally different from the dream imagery. The subject then believes that he/she has dreamed, but is unable to describe the dream.
The problem of somnambulism
Considered rare but not pathological, since it can occur in 10% of cases in children or adolescents under 15 years of age, somnambulism occurs during slow wave sleep, as proven by telemetric electroencephalographic recordings. A sleepwalking child is able to get up, open a door, and go get some food. When suddenly awakened, the sleepwalker does not know why he is up and has lost all memory of his episode.
Sleepwalking is thus a good example of the absence of psychoneural correlation that should remind the neurobiologist of his humility: indeed, despite the presence of slow cortical waves (and thus a contrario in the absence of cortical activation that we have considered as a condition of conscious attention), one can observe a complex behavior directed towards a goal. For a behaviourist, this behaviour could reflect an awareness similar to vigilant awareness. A more detailed analysis reveals that the memory of the sleepwalking behavior is absent.
At the end of this first analysis, neurobiology (and especially clinical neurophysiology) must admit that the relationship between sleep and consciousness is ambiguous and that the following provisional conclusions are plausible during slow wave sleep (which precedes the first dream phase), there is no evidence for the existence of reflexive consciousness — or consciousness of sleep — No one can say: I think that I am sleeping and even less: I think that I think that I am sleeping (see on this subject Sartre’s analysis of consciousness during sleep.
It is possible that the brief awakening triggered in a sleeper (and which is accompanied by cortical activation) is sufficient to allow the accession to a non-reflexive consciousness (I think I was thinking about something).
The perception of the sleepwalker, able to open and close a door, is a good example of unconscious perception. Sleepwalkers, even adult sleepwalkers, never tell themselves: I think I am walking in a somnambulistic state and they never remember their access when they are awakened.
Let us therefore admit that an unconscious perception not accompanied by mnemonic integration can sometimes exist in the absence of cortical activation during sleep.