The ontogenesis of sleep was studied in prematures from 32 to 41 weeks of gestational age The characteristics of the EEG, the patterns and rate of respiration, the cardiac rate, body motility comprised of localized and diffuse movements, localized and diffuse jerks, clonic movements and electromyo-graphic activity of the chin, as well as periods of total quietness were observed and correlated with the progressive organization of behavior in these infants. The data obtained at each gestational age, i.e., at 32–34 weeks, 35–37 weeks, and 38–41 weeks, were compared to those obtained in full-term newborns. The relationships between the different events and the features of the EEG and of the respiratory patterns were ascertained and compared to similar data obtained in the full-term newborn. This comparison shows that the premature at 40 weeks of gestational age has not attained a level of organization as high as that of the full-term newborn. An hypothesis suggesting a possible mechanism of this discrepancy is presented.