Mother-infant co-sleeping often accompanies nighttime breast-feeding. New research suggests that co-sleeping affects infant physiology and patterns of arousal, raising questions about currently accepted norms for "healthy" infant sleep.

Judging from the infant’s biology and evolutionary history, proximity to parental sounds, smells, gases, heat, and movement during the night is precisely what the human infant’s developing system "expects," since these stimuli were reliably present throughout the evolution of the infant’s sleep physiology. The human infant is born with only 25 percent of its adult brain volume, is the least neurologically mature primate at birth, develops the most slowly, and while at birth is prepared to adapt, is not yet adapted. In our enthusiasm to push for infant independence (a recent cultural value), I sometimes think we forget that the infant’s biology cannot change quite so quickly as can cultural child care patterns.

Infants sleeping for long periods in social isolation from parents constitutes an extremely recent cultural experiment, the biological and psychological consequences of which have never been evaluated. Most Americans assume that solitary sleep is "normal," the healthiest and safest form of infant sleep. Psychologists as well as parents assume that this practice promotes infantile physiological and social autonomy. Recent studies challenge the validity of these assumptions and provide many reasons for postulating potential benefits to infants sleeping in close proximity to their parents – benefits which would not seem likely with solitary sleeping. Current clinical models of the development of "normal" infant sleep are based exclusively on studies of solitary sleeping infants. Since infant-parent co-sleeping represents a species-wide pattern, and is practiced by the vast majority of contemporary peoples, the accepted clinical model of the "ontogeny" of infant sleep is probably not accurate, but rather reflects only how infants sleep under solitary conditions. I wonder whether our cultural preferences as to how we want infants to sleep push some infants beyond their adaptive limits.

To explore this possibility further, Dr. Sarah Mosko and I are studying the physiological effects of mothers and infants sleeping apart and together (same bed) over consecutive nights in a sleep lab. Our two pilot studies conducted at the University of California, Irvine School of Medicine, showed that the sleep, breathing, and arousal patterns of co-sleeping mothers and infants are entwined in potentially important ways. Solitary sleeping infants have a very different experience than social sleeping infants – although we do not know yet what our data mean.

Funded by the National Institutes of Child Health and Human Developments, this research will help us to evaluate the idea that infant-parent co-sleeping may change the physiological status of the infant in ways that, theoretically, could help some (but not all) SIDS-prone infants resist a SIDS event (McKenna 1986: McKenna et al. 1991: McKenna et al., in press). One of the suspected deficits involved in some SIDS deaths is the apparent inability of the infant to arouse to reinitiate breathing during a prolonged breathing pause. Our preliminary studies show that mothers induce small transient arousals in their co-sleeping infants at times in their sleep when, had the infant been sleeping alone, arousal might not have occurred. We have suggested that perhaps co-sleeping provides the infant with practice in arousing. Before we can draw any conclusions, more work is needed.

Regardless of what our own research will reveal, there already exists enough scientific information to justify rethinking the assumptions underlying current infant sleep research, as well as pediatric recommendations as to where and how all infants should sleep. Especially needed are new studies which begin with the assumption that infant-parent co-sleeping is the normative pattern for the human species – and that our own recent departure from this universal pattern could have some negative effects on infants and children. We need to determine if unrealistic parental expectations, rather than infant pathology, play a role in creating parent-infant sleep struggles – one of the most ubiquitous pediatric problems in the country. It may well be that it is not in the biological best interest of all infants to sleep through the night, in a solitary environment, as early in life as we may wish, even though it is more convenient if they did so.

Co-sleeping is often discussed as if it were a discrete, all-or-nothing proposition (i.e., should baby sleep with parents?). Many parents fail to realize that infants sleeping in proximity alongside their bed, or with a caregiver in a rocking chair, or next to a parent on a couch, in a different room other than a bedroom, or in their caregiver’s arms all constitute forms of infant co-sleeping. I studied the location of infants and parents in their homes between 6:00PM and 6:00AM and found more infant-parent contact than parents describe.

I prefer to conceptualize infant sleep arrangements in terms of a continuum ranging from same-bed contact to the point where infant-parent sensory exchanges are eliminated altogether, as, for example, infants sleeping alone in a distant room with the door closed. Nowadays, one-way monitors often broadcast infant stirrings to parents in these situations, compensating for the loss of sensory proximity.

I am amused by this baby monitor phenomenon, primarily because we Americans seem to have gotten it all backward. Rather than parents monitoring the infant, a great number of developmental studies suggest that it should be the other way around, with the infant processing parental stirrings (especially breathing sounds and vocalizations). Infant sleep, heart rate, breathing, and arousal levels are all affected by such stimuli, probably in adaptive ways to facilitate development and to maximize adjustment to environmental perturbations (Chisholm 1986). At the very least, monitors should be broadcasting sound in both directions!

Given the human infant’s evolutionary past, where even brief separations from the parent could mean certain death, we might want to question why infants protest sleep isolation. They may be acting adaptively, rather than pathologically. Perhaps these infant "signalers," as Tom Anders calls them, have unique needs and require parental contact more than do some other infants, who fail to protest. It’s worth considering.

James J. McKenna, Ph.D., is a Professor of Anthropology at Pomona College and Senior Researcher in the Department of Neurology, University of California, Irvine School of Medicine. Dr. McKenna and his colleague Dr. Sarah Mosko are funded by the National Institutes of Health and have been studying Infant-parent cosleeping: Implications for SIDS at the University of California, Irvine School of Medicine.