1) to answer the question "Do sleep states exist in normal premature infants;" 2) to analyze the development of sleep cycle and sleep state characteristics in premature and full-term newborns. Polygraph recordings were done on 38 normal, appropriate for gestational age newborns, born at 30 to 41 weeks (w) of gestation. All infants fell asleep in active sleep (AS). Postwaking AS was significantly shorter than the next AS. Mean sleep cycle duration increased from approximately 46 min at 31-34 w of conceptional age (CA) to 70 min. at 35-36 w CA. In all infants we observed stable, greater than 5 min AS and quiet sleep (QS) periods, as defined by EEG and REM criteria. Indeterminate sleep was about 30% of the total sleep cycle at 31-34 w; it decreased to 12% at 35-36 w. Both duration and percentage of AS and QS significantly increased at 35-36 w and remained stable up to 39-41 w CA. Values of QS were significantly reduced when defined by additional criteria (respiratory rate, tonic chin EMG or motility). Concordance of QS criteria was not significantly better in older versus younger groups of infants. At all ages, AS values were insensitive to changes in the criteria chosen to define them. The contrast, starting from 31-34 w CA, between AS and QS as defined by EEG and REM criteria could account for state differences in the control of many physiological variables in prematures.