Sleep-wake states and their regulatory mechanisms throughout early human development

The emergence of sleep states is one of the most significant aspects of development. Descriptions of both neonatal and late fetal behavior and studies on the organization of sleep have shown that fetus and newborns exhibit spontaneously discrete and cyclic patterns of active sleep (AS) and quiet sleep (QS). Human fetuses and neonates sleep most of their life, and AS is the prevailing state even during the first postnatal months. Several hypotheses to explain central nervous system development consider that AS is the expression of a basic activation program for the central nervous system that increases the functional competence of neurons, circuits, and complex patterns before the organism is called on to use them. Current results indicate the maturation of QS not only coincides with the formation of thalamocortical and intracortical patterns of innervation and periods of heightened synaptogenesis, since this sleep state is also associated with important processes in synaptic remodeling. In fact, several studies suggest that the information acquired during wakefulness is further processed during AS and QS. This article reviews the processes involved in the timing of both AS/QS and sleep/wake alternating patterns throughout early human development. A growing body of evidence indicates that the duration of unmodulated parental care and noncircadian environmental conditions may be detrimental for the establishment of these basic rhythmicities. As a consequence, alterations in parental/environmental entraining factors may well contribute to disturb sleep and feeding commonly experienced by preterm infants. Further knowledge on the early establishment of sleep-wake states regulatory mechanisms is needed to improve modalities for appropriate stimulation in the developing human being.