EEG polygraph recordings of 1 to over 4 h duration were obtained during daytime sleeps weekly from birth to 11-13 weeks of age in 17 normal full-term newborns. Analysis of the recordings permitted more precise specification of the time courses of early developmental changes in EEG patterns related to the sleep cycle. The tracé alternant pattern of quiet sleep was seen up to 2 weeks post term in all subjects, but in none beyond 6 weeks. Active sleep onset occurred in 80% of daytime sleeps at 1-3 weeks and decreased rapidly over the next 5 weeks, but at 8-13 weeks was still seen in 5-10% of the recordings. Rolandic sleep spindle bursts appeared in some subjects as early as 4 weeks post term and were present in all beyond 8 weeks. Based upon these 3 criterion variables, the transition from 'perinatal' to 'infantile' EEG sleep patterns started at a mean age of 30.3 days and was completed at a mean age of 46.6.days. The earliest and latest completions of the transition were at 27 and 66 days, respectively. On the average the transition took just over 2 weeks. Frontal sleep transients were seen until 3 weeks post term in all subjects, but in none beyond 7 weeks. Active sleep decreased from just over 50% of total sleep time at birth to about 20% beyond 8 weeks. Percent-time quiet sleep increased proportionately. Indeterminate sleep remained relatively constant at 5-10 percent-time. A tendency for percent-time indeterminate sleep to be elevated in the presences of minor illnesses was observed. The usefulness of these, and other, EEG data in defining mature and immature EEGs in the neonatal period and the clinical significance of EEG immaturity are discussed.