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Mothers and infants sleeping side by side, also known as co-sleeping, is the evolved context of human infant sleep development. Until very recent times, for all human beings, co-sleeping constituted a prerequisite for infant survival. For the majority of contemporary people outside of the Western industrialized context, it still does. Because the human infant’s body continues to be adapted only to the mother’s body, co-sleeping with nighttime breastfeeding remains clinically significant and potentially lifesaving.
This is because, of all mammals, humans are born the least neurologically mature (25% of adult brain volume), develop the most slowly, and are the most dependent for the longest period of time for nutritional, social, and emotional support, as well as for transportation. Indeed, in the early phases of human infancy, social care is synonymous with physiological regulation. That is, holding, carrying, and/or caressing an infant, and emitting odors and breath in his or her proximity, induce increased body temperature, less crying, greater heart-rate variability, fewer apneas, lower stress levels, increased glucose storage, and greater daily growth.1
Moreover, since the content of human milk is relatively low in fat and protein and high in sugar, which is metabolized quickly, and since human infants are unable to locomote on their own, continuous contact and carrying, with frequent breastfeeding day and night, is required. Thus, any biological scientific study that attempts to understand “normal,” species-wide, human infant sleep patterns without considering the vital role of nighttime contact in the form of breastfeeding and maternal proximity must be considered inadequate, misleading, and/or fundamentally flawed.2
Co-sleeping: The Importance of Taxonomic Distinctions
Much of the controversy surrounding the question of the safety of mother– infant co-sleeping involves the ways in which investigators define and conceptualize it. Co-sleeping is not, as the Consumer Product Safety Commission (CPSC) assumes, a single, coherent practice. Rather, it is best thought of as a generic, diverse class of sleeping arrangements composed of many different practices, each of which requires proper description and characterization before the issue of safety and clinical outcomes can be understood.
A safe co-sleeping environment must provide the infant with the opportunity to sense and respond to the caregiver’s signals and cues, such as the mother’s smells, breathing sounds and movements, infant-directed speech, invitations to breastfeed, touches, and any hidden sensory stimuli, whether intended or not.3 Moreover, to be designated as safe, the physical and social co-sleeping environment must involve a willing and active caregiver who chooses to co-sleep specifically to nurture, feed, or be close to the infant in order to monitor or protect him or her.