Rapid eye movements occur in the normal newborn when eyes are closed (sleep-REM), when eyes are open (drowsy-REM), and during some portion of the time when there is sucking, fussing, or crying (sucking-REM, fussy-REM, and crying-REM). In an electroencephalographic (EEG)-polygraphic study of these behavioral states, it was found that they are all associated with stage-REM physiology. Although the states can reliably be distinguished from one another using behavioral criteria alone, they cannot easily be distinguished from one another on the basis of isolated study of electrophysiological recordings. Independent behavioral and EEG-polygraphic assessments show high agreement in distinguishing these states as a group from other sleep and nonsleep states. The term “undifferentiated” is applied to drowsy-REM, sucking-REM, fussy-REM, and crying-REM because their behavioral and physiological indices are relatively unpatterned and, from the viewpoint of infant development, these states disappear during the first 3 months of postnatal life. Neonatal REM sleep is also conceptualized as undifferentiated as it shows an initial high variability of physiological patterning which tends toward stability over the first 3 months. This change is concomitant with another major change: at 3 months, sleep begins with non-rapid eye movement sleep instead of REM sleep.