Intrinsic and extrinsic factors predicting infant sleep: Moving beyond main effects

Sleep patterns change dramatically across the earliest years of life and play an important role in children's daytime functioning. As a result, psychological research has taken an increasing interest in unpacking the many intrinsic (i.e., child characteristics) and extrinsic (i.e., environmental input) factors that influence children's sleep development. Considerably less attention has been given to understanding the transactional relationships among intrinsic and extrinsic factors, or to the underlying mechanisms, that both initiate and maintain individual differences in infant sleep development. In the current review, we begin by summarizing what is known about the development of sleep across the first two years of life, making explicit reference to the dual process model of sleep consolidation and regulation. Next, we synthesize existing literature on the intrinsic and extrinsic factors that influence the development of sleep consolidation and regulation in infants. Finally, we reconsider extant findings in the infant sleep literature using theories and concepts from developmental science, posing new hypotheses about the ways in which environmental input both shapes infant sleep patterns and modulates the effects of sleep on later developmental outcomes. We conclude with an examination of current challenges in this field and a suggested roadmap for future research.