Post-natal development of sleep organization in man: speculations on the emergence of the 'S process'.

We outline the development during the first year of life of both sleep-wake patterns and internal sleep organization (sleep states, quiet sleep-paradoxical sleep cycles, and slow-wave sleep), and raise the issue of the emergence of the 'S process'. The consolidation of sleep in episodes of longer duration takes place during the night; the ability to maintain sleep after sleep onset decreases during the day in infants older than 6 months. The main contribution to the increase of longest nocturnal sleep episode duration comes from the quiet sleep, the number of quiet sleep-paradoxical sleep cycles being of minor importance. The temporal analysis of infant sleep structure showed a difference between the first and the following cycles, not only for slow wave activity in infants older than five months, but also for younger infants and for other individual physiological activities (EEG activity, respiratory and cardiac rates): is the early development of these peculiar features a sketch of the 'S Process' described in the adult?