Full length articleExperimental manipulation of maternal proximity during short sequences of sleep and infant calming response
Introduction
This study investigated the effect of mother-infant co-sleeping (CS), infant sleeping beyond arms length of the mother (BAL), and infant solitary sleeping (SS) on infant physiological calming, a self-regulatory process controlled by the parasympathetic pathway of the autonomic nervous system. The effects of all three prevalent sleeping conditions on infant physiological calming were assessed amongst infants who regularly co-slept, infants who regularly slept in the BAL sleeping arrangement, and in infants who slept solitarily. Mother and infant co-sleeping is a parenting custom that has endured through perhaps five million years of human history (Ball, 2003). This sleeping arrangement occurs when mother and infant share the same surface (i.e., bed-sharing) in close enough proximity for the two to respond to one another’s cues (Huang, Wang, Zang, & Liu, 2010) and engage in bodily contact during the night (Lozoff, Askew, & Wolf, 1996). Only in the last two centuries has this traditional practice been questioned, and the idea of alternative sleeping arrangements surfaced. This paradigm shift was driven in part by changing cultural norms, foremost of which is the rise of the modern Western thought that places great emphasis on the development of personal independence (Keller & Goldberg, 2004a). Today, sleeping arrangements alternative to co-sleeping consist of solitary sleeping, where the infant sleeps separately from the mother (i.e., in another room), and room-sharing, in which the infant remains in the vicinity of the mother but is not within the mother’s reach (i.e., beyond arm’s length). As suggested, cultural norms greatly influence infant sleeping arrangements. Starkly different from Western nations (such as the United States) that typically adopt solitary sleeping arrangements thought to promote independence in infants (Keller & Goldberg, 2004a) are national norms (such as in Japan and China), where, respectively, mothers feel that solitary sleep is “merciless” (Morelli, Rogoff, Oppenheim, & Goldsmith, 1992) and mothers think that co-sleeping is critical to infant security (Huang et al., 2010). In a cross-cultural study that compared Western and Asian countries, the former reported higher rates of solitary sleeping, whereas the latter predominantly practiced co-sleeping (Mindell, Sadeh, Kwon, & Goh, 2013). Despite the spread of Western ideas among Asian societies that galvanized sociodemographic change (e.g., women’s participation in the workforce, urbanization), Shimizu, Park, and Greenfield (2014) found equal prevalence of co-sleeping in Japan in 1980s as compared to the present, explained mostly by societal expectations of mothers, limited space, or greater emphasis on cultural values. Mahendran, Vaingankar, Mythily, and Cai (2006) also found that 33.1 % of children aged 2–19 years still slept with their parents in Singapore which is an economically advanced society, like Japan.
The ongoing debate about sleeping arrangements derives from contrasting opinions about the advantages and disadvantages of co-sleeping and has consequently spurred much research interest. Some studies support the benefits of co-sleeping, such as for comfort and warmth (Esposito, Setoh, & Bornstein, 2015; Esposito, Setoh, Yoshida, & Kuroda, 2015), better accessibility to cater to infant needs, such as breast-feeding and night-awakenings (Ball, 2003; Mao, Burnham, Goodlin-Jones, Gaylor, & Anders, 2004; McCoy et al., 2004; Mileva-Seitz, Bakermans-Kranenburg, Battaini, & Luijk, 2017; Mosko, Richard, & McKenna, 1997), and enhanced parent-child bonding (Chu, 2014; Esposito, Setoh, Bornstein et al., 2015; Esposito, Setoh, Yoshida et al., 2015; Trevathan & McKenna, 1994). Conversely, others espouse solitary sleeping and warn about the dangers of co-sleeping, such as increased risk of infectious diseases (McCallion et al., 1996; Mileva-Seitz et al., 2017) and Sudden Infant Death Syndrome (SIDS; McKenna & McDade, 2005; Moon, 2011; Shimizu et al., 2014). The belief in allegedly promoting infant autonomy also drives both types of parenting practices. Infant solitary sleep is thought to foster infant self-soothing behaviors, self-reliance, and later independence (Burnham, Goodlin‐Jones, Gaylor, & Anders, 2002; Keller & Goldberg, 2004a; McKenna & Mosko, 1994; Mileva-Seitz et al., 2017). Co-sleeping is likewise believed to promote independence due to enhanced mother-child bonding at night that supports the child to become more autonomous during the day (Chu, 2014; McKenna & McDade, 2005).
Autonomy begins with the infant’s physiological and psychological independence from the mother (Shimizu et al., 2014; Winberg, 2005). Distinct preferences of sleeping arrangements across cultures reflect differences in opinion as to when independence is presumed to develop. For instance, proponents of solitary sleep in the United States view birth as the point of physiological separation of the mother and child, after which infants are encouraged to further develop independent self-regulatory abilities (i.e., self-soothe to sleep at night; Burnham et al., 2002; Keller & Goldberg, 2004a; McKenna & Mosko, 1994; McKenna et al., 1994). However, infant physiology is immature and normally requires extensive external support from the mother for effective regulation (Winberg, 2005, McKenna and Mosko (1994) established that physical contact with the mother allows for the continuous exchange of dyadic sensory cues (i.e., motion, vocalization, touch, etc.) that provides external support to the child as the child gradually adapts and develops self-regulatory capacities (e.g., Baddock, Galland, Beckers, Taylor, & Bolton, 2004; Ball, 2006; Richard & Mosko, 2004). In the context of co-sleeping, the close proximity of mother and child promotes skin-to-skin contact that regulates the infant’s core body temperature, enhances glucose metabolism and reduces cortisol levels (Lester et al., 2018; McKenna & Volpe, 2007; Moore et al., 2017; Winberg, 2005). These findings seem to contradict with the prevailing notion of infant physiological post-birth independence held by parents who advocate solitary sleep.
An indication of infant physiological independence is the ability to engage in effective physiological self-regulation, which is the maintenance of a constant internal homeostatic autonomic state of arousal (Pace-Schott et al., 2019; Porges, Doussard-Roosevelt, Portales, & Greenspan, 1996; Ramsay & Woods, 2014). Because self-regulation processes involve cortical areas, such as the prefrontal and frontal areas, that communicate with and control the autonomic nervous system, the rMSSD (root of the Mean of the Square of Standard Deviation), which is a measure of heart rate variability (HRV) has been identified as an index of self-regulatory capacity (Porges, 2001; Reynard, Gevirtz, Berlow, Brown, & Boutelle, 2011; Segerstrom & Nes, 2007; Thayer & Lane, 2000). HRV is the fluctuation in the interval time between two adjacent heartbeats or interbeat intervals (IBI) and reflects the activity of the parasympathetic network of the autonomic nervous system (Shaffer & Ginsberg, 2017; Task Force, 1996). Longer IBI and greater HRV reflect a state of calming mediated by the parasympathetic pathway involved in self-regulation, whereas shorter and lower indices, respectively, denote physiological arousal due to increased activation of the sympathetic nervous system, often involved in the stress response (Esposito et al., 2013; Esposito, Setoh, Bornstein et al., 2015; Esposito, Setoh, Yoshida et al., 2015; Reynard et al., 2011; Stern, Ray, & Quigley, 2001; Webber et al., 2010). More mature and developed autonomic systems would exhibit longer IBI indices that suggest effective physiological self-regulation (Evans et al., 2013; Longin, Gerstner, Schaible, Lenz, & König, 2006; Schneider et al., 2018).
At present there is scarcity in the literature as to how mother-infant sleeping arrangements impact infants’ self-regulation, particularly their calming response. Understanding the effect of different types of prevalent mother-infant sleeping arrangements (bed-sharing, room-sharing, and solitary sleeping) on physiological calming responses, measured by IBI and HRV, through systematic manipulation of sleeping arrangements would provide better insight into which sleeping arrangement could be beneficial to establish calming and promote greater self-regulation in infants. Taking into account the infants’ habitual sleeping arrangement -- infants who regularly co-sleep (CS) with their mothers, sleep beyond arm’s length (BAL) from their mothers, and sleep solitarily (SS) -- this study assessed infant physiological calming and self-regulation in each of these three sleeping arrangement condition during a 30 min nap. IBI and HRV were measured as physiological responses. Stress, as opposed to calming, shortens IBI and lowers HRV, whereas self-regulatory effort elevates IBI and HRV (Esposito, Setoh, Bornstein et al., 2015; Esposito, Setoh, Yoshida et al., 2015; Reynard et al., 2011; Segerstrom & Nes, 2007). Due to the stress induced by mother-infant separation, we hypothesized that infants would display greatest physiological calming during co-sleeping, less in the BAL sleeping condition, and show physiological stress in the solitary sleeping condition (Feldman, Singer, & Zagoory, 2010; Keller & Goldberg, 2004b; Morgan, Horn, & Bergman, 2011; Pryce, Aubert, Maier, Pearce, & Fuchs, 2011). Additionally, due to enhanced mutual regulation and intimacy when dyadic partners are in close physical contact (Ball, Hooker, & Kelly, 2000; Chu, 2014; Keller & Goldberg, 2004a, 2004b, McKenna & Mosko, 1994; McKenna & McDade, 2005), we expected infants who regularly co-sleep would have developed greater physiological self-regulation and adapt better to changes in sleeping conditions compared to their counterparts who regularly experience BAL and solitary sleeping arrangements. Alternatively, if regular solitary sleeping promotes self-regulation in infants (Burnham et al., 2002; Keller & Goldberg, 2004a; McKenna & Mosko, 1994; McKenna et al., 1994), physiological self-regulatory responses may also be found in infants who are regular solitary sleepers.
Section snippets
Participants
Twenty-four mother-infant dyads from Singapore (Chinese Ethic group) took part in this study. The infants (50% female) were aged between 1–3 months (M = 1.85 months SD = 0.93 months). All the infants were born full-term with no health issues. The age of the mothers ranged from 23 to 42 years (M = 32.26 years, SD = 3.941). Most mothers were primiparous, except three who had one other older child. All mothers were currently married, had a form of tertiary education, and were healthy with no
Effect of sleeping arrangements on infant IBI
A 3 × 3 RM-ANOVA was conducted to assess the effect of mother-infant sleeping arrangement on the mean interbeat interval in infants who regularly co-sleep, sleep beyond arms length from their mother, or sleep solitarily. There was a significant interaction between sleeping arrangement condition and the type of infant regular sleeping arrangement, F(2.30, 196.82) = 41.62, p < 0.001, partial η2 = 0.327. Post-hoc analyses revealed statistically significant differences in mean IBI during the
Discussion
This study investigated the effects of three mother-infant sleeping arrangement (co-sleeping, sleeping beyond arm’s length, and solitary sleeping) conditions on infant physiological calming and self-regulation as reflected by infants’ mean heart rate interbeat interval and heart rate variability in infants who are regular co-sleepers, beyond arm’s length sleepers, and solitary sleepers. Specifically, each infant of each sleeping type was observed in all three sleeping arrangements conditions in
Authors statement
Conceptualization, G.E. and P.S.; data collection, B.L.R. and A.A.; data analysis, BLR.; writing—original draft preparation, B.L.R.; writing—review and editing, A.A., M.H.B. and G.E.. All authors have read and agreed to the final version of the manuscript.
Acknowledgements
This research was supported by NAP SUG 2015 (GE), Singapore Ministry of Education ACR Tier 1 (PS and GE), Social Science Research Thematic Grant (MOE2016-SSRTG-017, PS), the Intramural Research Program of the NIH/NICHD, USA (MHB), and an International Research Fellowship at the Institute for Fiscal Studies (IFS), London, UK (MHB), funded by the European Research Council (ERC) under the Horizon 2020 research and innovation programme (grant agreement No 695300-HKADeC-ERC-2015-AdG).
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