Evolutionary Perspectives on Mother–Infant Sleep Proximity and Breastfeeding in a Laboratory Setting

Mother–infant cosleeping is defined as any situation in which a committed mother sleeps within sensory range of an infant (on the same or different surface) permitting mutual monitoring, sensory access, and physiological regulation, including (but not limited to) the delivery and ingestion of breast milk (McKenna et al., 1993). Such close mother–infant proximity during sleep is common across cultures and is nearly ubiquitous among primates (Barry and Paxson, 1971; Anderson, 1984; McKenna et al., 2007). Compared with most other primates, humans are especially vulnerable and dependent, being born with only 25% of adult brain volume, as a result of the anatomical limitations imposed by encephalization and bipedality (Rosenberg and Trevathan, 2005; Robson et al., 2006; Martin, 2007). Consequently, it is highly probable that infants slept near or on their mothers' body to be fed and nurtured over the course of hominin evolution, although evolutionary reconstructions will never be able to fully prove the point, given that maternal behavior does not fossilize or leave an otherwise indelible mark in the archaeological record. Still, whether during wake or sleep, maternal proximity and contact likely maximized the chances of the infant acquiring adequate safety, nutrition, and immunological protection in the challenging ecologies facing our hominin ancestors (McDade 2003; Hart and Sussman, 2005; McKenna et al., 2007). An anthropological perspective, largely missing in contemporary pediatric sleep research (Anders et al., 1971; Guilleminault and Souquet, 1979; Ferber and Kryger, 1995; Lee-Chiong, 2006), draws on cross-cultural, cross-species, and developmental data to model evolved human norms vis-a-vis mother–infant sleep behaviors. This explanatory framework illustrates how and why maternal breast milk delivery and metabolism (by the infant) combined with mother–infant cosleeping changes almost every important clinical sleep variable for both the mother and infant in potentially positive ways when compared with data derived from solitary sleeping, bottle-fed infants (McKenna et al., 2007). This was initially demonstrated when the first ever study of the behavior and physiology of mother–infant cosleeping was proposed and undertaken 20 years ago to evaluate the appropriateness of conceptualizing solitary sleep as “normative” or “healthy” for the human infant (McKenna et al., 1990). Subsequently, numerous scholars have applied anthropological and evolutionary perspectives to questions of mother–infant sleep practices, especially as related to breastfeeding patterns and health and safety issues in the bedsharing (cosleeping on the same surface in a bed) environment (reviewed in McKenna et al., 2007; Ball and Klingaman 2008). The extent of comparative physiological and behavioral differences between solitary sleeping and cosleeping infants suggests that a significant, possibly harmful, disjuncture may emerge between behavior and biology when solitary infant sleep practices are favored, giving rise to evolutionarily anomalous experiences. For example, not breastfeeding is now known to be an independent risk factor for sudden infant death syndrome (SIDS) (Vennemann et al., 2009), as is an infant sleeping alone in a room by itself (Mitchell and Thompson 1995; Blair et al., 1999; Carpenter et al., 2004). Although a growing body of evidence from studies in the US, UK, and New Zealand (McKenna et al., 1997; Young 1999; Ball, 2003; McCoy et al., 2004; Ball et al., 2006; Baddock et al., 2007; Ball, 2007) and subsistence-level populations (Super and Harkness 1987; Imong et al., 1989; Sellen, 2001) demonstrate that mother–infant sleep practices relate to day-to-day and overall, long-term breastfeeding patterns, studies examining the contextual factors that influence breastfeeding outcomes generally fail to include sleep location as a relevant variable (e.g., Arora et al., 2000; Alikassifogglu et al., 2001; Ertem et al., 2001; Scott et al., 2001). This may owe, at least in part, to a general paucity of observational, experimental data on the ways in which mothers and infants interact and relate to each other during sleep periods, especially as regards factors influencing nighttime feeding behaviors. While three important studies shed further light on these issues (Young, 1999; Ball et al., 2006; Baddock et al., 2007), a major lacuna still exists and we elaborate on previous work here to help fill it. To explore the relationship between breastfeeding behavior and routine sleeping arrangements we examine data from three consecutive nights of observed breastfeeding activities, recorded on infrared video in a controlled, laboratory setting. Previous analyses of breastfeeding behavior in this sample compared feed frequency for only the second and third nights in the laboratory and did not assess feed interval data for any night (McKenna et al., 1997). Hundreds of hours of recordings that include data on potential differences and similarities in breastfeeding patterns between solitary infant sleeping and bedsharing were left unanalyzed. The present analysis focuses on the first night data (n = 26, infant age 12.5 ± 2.5 weeks) alone and in combination with the second and third night data (n = 36, age 12.5 ± 2.4 weeks) to test the hypotheses that during nighttime sleep routinely bedsharing mother–infant dyads engage in significantly more breastfeeds and show a shorter interval duration between feeds when compared with routinely solitary sleeping pairs.