This research will examine one aspect of maternal-infant interaction, mother-infant adaption in the infant's sleep behavior, and its relationship to the infant's mental and psychomotor development. Adaption in the mother-infant system will be measured in terms of regularity of infant sleep behaviors, and the level of stress experienced by the mother with reference to sleep behavior and parenting in general. Subjects will be 25 adolescent (16-18 years old) and 25 adult single mothers (20-35 years old) and their first-born infants. Groups will be comparable in socioeconomic status, adequacy of prenatal care, and absence of major perinatal complications. Infant sleep behavior will be assessed by direct observation in the home at 3 and 5 weeks, and by mothers' reports at 4, 8, and 12 months. Degree of maternal stress over sleep and other parenting issues will be assessed by interview, diary, and questionnaire at each time period, and the child's developmental status will be assessed at 12 months. Results will be examined to determine 1) whether infants of adolescent mothers show less regularity of sleep behavior than infants of adult single mothers; 2) whether adolescent mothers show higher levels of stress related to sleep and parenting in general; 3) whether and how these measures of the elements of the system are related to each other across time; and 4) how they relate to the child's developmental status at 12 months.