[Paradoxical sleep: is it the guardian of psychological individualism]

The brain is the sole organ of homeotherms that do not undergo cell division. We thus have to explain how certain aspects of psychological heredity (found in homozygotes twins raised in different surroundings) may persist for a whole life (psychological individuation). A definitive genetic programming during development (by neurogenesis) is unlikely due to the plasticity of the nervous system. That's why we have to consider the possibility of an iterative genetic programming. The internal mechanisms (synchronous) of paradoxical sleep (SP) are particularly adapted to such programming. This would activate an endogenous system of stimulation that would stimulate and stabilize receptors genetically programmed by DNA in some neuronal circuits. The excitation of these neurons during SP leads to oniric behaviours that could be experimentally revealed--the lists of these behaviours are specific to each individual and indirect data suggest a genetic component of this programming. Amongst the mechanisms allowing the iterative programming of SP, sleep is particularly important. Security--and hence the inhibition of the arousal system--is a sine qua non condition for genetic programming to take place. In that sense, sleep could very well be the guardian of dreaming. On the other hand, sleep seems to be necessary for the accumulation of energetic resources used by the cholinergic mechanisms of SP. The temporal modalities of SP (diachronic organization) are also discussed in relation to phylogenesis. Thus, the absence of SP in poikilotherms is explained by a continual neurogenesis in the adult. During ontogenesis in mammals, a stage of programming by neurogenesis (seismic sleep) precedes the appearance of SP so long as the programming system isn't functional. The presence, or absence, of rebound after SP deprivation is interpreted in terms of the existence, or non existence, of stress during SP suppression. An explanation is proposed to account for the absence of specific effects of SP deprivation in humans. In the same way somatic intraspecific variability is one of the conditions of evolution, it is proposed that one of the functions of SP is to maintain psychological variability in a given population.