Why we sleep: the evolutionary pathway to the mammalian sleep

The cause of sleep is a complex question, which needs first, a clear distin ction 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 po lygraphic sleep. This review aims at clarifying the meaning of the question and at showing the phylogenetic origin of the mammalian and avian sleep. T he phylogenetic appearance of sleep can be approached through a study of th e evolution of the vertebrate brain. This began as an undifferentiated dors al nerve, which was followed by the development of an anterior simplified b rain and ended with the formation of the multilayered mammalian neocortex o r the avian neostriate. The successive stages in the differentiation of the vertebrate brain produced. at least, two different waking types. The oldes t one is the diurnal activity, bound to the light phase of the circadian cy cle. 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 system s, the early waking type, controlled from the brainstem, was suppressed, an d 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 shou ld thus be considered an evolutionary remnant: the true acquisition of mamm als is the cortical waking and not the sleep. (C) 2000 Elsevier Science Ltd . All rights reserved.