Why we sleep: the evolutionary pathway to the mammalian sleep

The cause of sleep is a complex question, which needs first, a clear distinction amongst the different meanings of a causal relationship in the study of a given behavior, second, the requisites to be met by a suggested cause, and third, a precise definition of sleep to distinguish behavioral from polygraphic sleep. This review aims at clarifying the meaning of the question and at showing the phylogenetic origin of the mammalian and avian sleep. The phylogenetic appearance of sleep can be approached through a study of the evolution of the vertebrate brain. This began as an undifferentiated dorsal nerve, which was followed by the development of an anterior simplified brain and ended with the formation of the multilayered mammalian neocortex or the avian neostriate. The successive stages in the differentiation of the vertebrate brain produced, at least, two different waking types. The oldest one is the diurnal activity, bound to the light phase of the circadian cycle. Poikilotherms control the waking from the whole brainstem, where their main sensorymotor areas lie. Mammals developed the thalamocortical lines, which displaced the waking up to the cortex after acquiring homeothermy and nocturnal lifestyle. In order to avoid competence between duplicate systems, the early waking type, controlled from the brainstem, was suppressed, and by necessity was turned into inactivity, probably slow wave sleep. On the other hand, the nocturnal rest of poikilotherms most probably resulted in rapid eye movement sleep. The complex structure of the mammalian sleep should thus be considered an evolutionary remnant; the true acquisition of mammals is the cortical waking and not the sleep.