Serotoninergic neurons play a critical role in the sleep mechanism. This is supported by a lot of converging experiments and has provided the basis for a great deal of research. A critical analysis is first developed, supported by more recent data which are not in complete agreement with the theory that raphe nuclei are actively implied in slow wave sleep. On the other hand, numerous experimental evidences were collected during the sixties on the EEG synchronizing influence of the lower brain stem and preoptic area. Recent data showed that serotonin could also play here a crucial role in the induction of sleep. Nevertheless, at the moment, it is difficult to make a critical examination of the interaction and regulation of these putative 5-HT-related areas of the brain, but we can postulate that the occurrence of true physiological sleep-waking continuum necessitates their successive or conjoint activation.