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The adaptation of the infant to the twenty-four hour periodicity of night and day through a gradually synchronized patterning of sleep and wakefulness, detectable in the third to fourteenth week of life, may represent an earlier function of the ego than the previously described third-month phenomena of the smiling response and purposeful finger sucking. The differentiation of a sleep-wakefulness pattern—the tendency of all living matter to develop a rhythmicity of rest and activity—represents a precursor of ego functioning in so far as it reflects the modification of an instinctual need by the reality principle. In addition it represents a convenient background against which to align the developmental landmarks described in physiological, psychological, and biochemical terms by the various disciplines studying child development. More importantly, this rhythm is the summation, in a single, semiquantitative measure, of such interrelated phenomena as innate motility patterns, perceptual responsiveness, sensorimotor integration, and the increasing influence of the environment. The observations by Kleitman and Engelmann of the sleeping, waking, and feeding patterns of normal infants in the first twenty-six months of life allow a rough delineation of three developmental phases in this time span. Both the ego and the sleep rhythm evolve from an 'undifferentiated phase' (from birth to the third week when manifest behavior, homeostatic regulations, and congenital ego tendencies are inseparable), through a 'phase of ego precursors' (third to twelfth or fourteenth week, in which the systematic adaptation of the sleep-wakefulness pattern reflects a primitive awareness of time as a function of outer reality represented by the diurnal variations in the quantity and quality of the mothering response), to the 'phase of beginning object relations' (from the twelfth or fourteenth week to the twenty-sixth week). In this latter phase the infant shows a rapid advance in perceptual