Original cartoon by Alex Martin
Source: Original cartoon by Alex Martin

For me, a moment of truth has arrived. The express goal of this blog — as for my book How We Do It — is to explore the evolution of human reproduction to uncover basic principles. This can yield vital contributions to evolutionary (Darwinian) medicine. Yet pervasive cultural influences often obscure underlying biological factors, and in many ways we have lost our compass for navigating the reproductive maze. Successful recognition of basic biological principles, despite all those cultural effects, can yield answers to questions such as “Does a woman need to breastfeed her baby, and if so for how long?” Any answers that emerge are generally likely to bring benefits with little risk of causing harm.

But the question “Should a mother sleep with her baby?” is in a class of its own; answering it may literally have life-or-death consequences. Note the key distinction, though. between bed-sharing and “co-sleeping”, which includes any arrangement with the baby within arm’s reach of the mother, as in a crib alongside the parental bed. Here, I focus on actual bed-sharing. To be honest, I previously dodged this issue because I could not confidently provide risk-free comments. Recent findings have tipped the balance and I am now convinced that bed-sharing is not only entirely natural but also beneficial and safe under appropriate conditions.

Deep Roots for Mothers Sleeping with Infants

Given the sometimes acrimonious debate about human bed-sharing, astoundingly co-sleeping mothers and babies have barely been studied in other primate species. For higher primates (monkeys and apes) much has been written about mother-infant interactions during daytime, but exceedingly little about behaviour at night. The universal rule is that by day infants are carried around by clinging to an adult, usually the mother but sometimes the father or another individual. Young babies are carried almost continuously, but as they mature they increasingly venture away until reaching independence. By contrast, during night-time sleep any unweaned infant clings tightly to its mother, usually high up in a tree. Even species that spend much of the day on the ground usually climb trees or other sheltered places such as cliff faces to sleep. Monkeys or apes in sleeping huddles surely have special adaptations to prevent them — and especially their babies — from falling to the ground or suffocating.

Original illustration by Nancy Klaud
Hypothetical reconstruction of the common ancestor of primates, some 80 million years ago
Source: Original illustration by Nancy Klaud

Carrying infants is a universal feature of primates and is also seen among prosimians (lemurs, lorises, tarsiers). In fact, my own interpretation is that the birth of a single infant carried by the mother probably characterized ancestral primates, some 80 million years ago. Although exceptions exist — such as a few nest-using prosimians that give birth to two or more babies occasionally carried around in the mother’s mouth — close mother-infant contact throughout the sleeping period is universal. Attachment parenting with co-sleeping has an 80-million-year pedigree! While our feet lost their grasping capacity during our adaptation for upright walking and our covering of body hair also disappeared, the fundamental mother-infant bond was surely enhanced, not reduced, during human evolution. Suckling on demand, rather than on a schedule set by the mother, was an integral part of that bonding.

Sudden Infant Death

The unexpected, symptomless death of a human infant — first officially recognized in 1969 as Sudden Infant Death Syndrome (SIDS) — is a devastating tragedy. It is particularly traumatic because parents tend to blame themselves. SIDS peaks between the second and fourth months after birth, but despite intensive research its causes remain unknown, and it is defined by default as deaths during the first year for which there is no identifiable cause such as infection. However, certain findings are now widely accepted. It is well established that SIDS is far more common when a baby sleeps lying face down (prone). The American Academy of Pediatrics (AAP) officially recognized this in 1992, recommending that babies should sleep lying on their backs (supine). Over the next 15 years, the frequency of supine sleeping increased from 13% to 76% and the incidence of SIDS declined by more than half.

Figure redrawn from Task Force on Sudden Infant Death Syndrome (2011)
Occurrence of Sudden Infant Death Syndrome during the first year after birth
Source: Figure redrawn from Task Force on Sudden Infant Death Syndrome (2011)

Because a baby’s sleeping position clearly influences the likelihood of SIDS, it is understandable why many regard mother-infant bed-sharing as potentially dangerous. It is widely believed that “overlying” by the mother is a common cause of early infant death. And several authors have reported that SIDS is more frequent with bed-sharing infants than with babies left to sleep separately. This frightening prospect is clearly a major deterrent for many parents, and the AAP currently recommends that a baby should sleep in the same room as the mother, but not in the same bed. Regrettably, some authorities have gone further to demonize bed-sharing. A particularly shocking example is a current campaign by the City of Milwaukee Health Department, with lurid posters proclaiming that bed-sharing kills. The website states that 48 infants died of SIDS or accidental suffocation after discharge from hospital in Milwaukee in 2009-2011. Of these 32 “were sharing a bed with parents, other children, or a caregiver when they died.” Yet that information says nothing. If 67% of families in Milwaukee practice some form of bed-sharing, 32 SIDS cases would be expected by chance alone!

A contrastingly well-supported finding elsewhere is that the risk of SIDS is greater with formula-feeding than with breastfeeding. One striking example is a report by Mechtild Vennemann and colleagues. Their analysis compared more than 300 infants who died of SIDS with 1,000 age-matched controls. With exclusive breastfeeding at one month of age the risk of SIDS was halved throughout infancy.

Mother-Baby Sleep Research

In contrast to the sparse information for nonhuman primates, quite a lot is known about human mothers sleeping with babies, especially thanks to the efforts of James McKenna, who now directs the Mother-Baby Sleep Laboratory at Notre Dame University. A decade after completing his Ph.D. on social behaviour of langurs (Indian monkeys) in 1975, he initiated the first ever research on sleep in human mother-infant pairs. He and his colleagues, notably Sarah Mosko (a clinical psychologist) and former Ph.D. student Lee Gettler, have since accumulated a wealth of objective data on co-sleeping. Studies have included recordings of breathing, heart rate, eye movements and brain waves under different conditions, with infants sleeping alone in an adjacent room, sleeping in the same room but in a separate crib or sleeping in the mother’s bed. One key early finding is that transient arousals are markedly synchronized in bed-sharing mother and babies. Breathing pauses (apneas) occur in all babies several times during the night. If a pause is over-long, a healthy infant will wake up to breathe, and repeated arousals by the mother may entrain unaided waking. Research has indicated that babies prone to SIDS may have some deficiency in their arousal mechanism that makes them less likely to wake up if they stop breathing.

Adapted from McKenna et al. (2007)
Frequency of breastfeeding attempts and successes with bed-sharing, infant in crib in same room or in a cot in a separate room
Source: Adapted from McKenna et al. (2007)

One major finding reported by McKenna and colleagues virtually from the outset is that bed-sharing increases the frequency and duration of breastfeeding. Mothers sleeping with babies breastfeed more often, with shorter intervals, throughout the night. Other research groups have independently confirmed that bed-sharing promotes breastfeeding. A 2010 paper by Peter Blair and colleagues provided strong evidence for an association between longer duration of bed-sharing and increased persistence of breastfeeding, and very similar results were reported by Li Huang and colleagues in 2013. In fact, in a recent review of key evidence McKenna and Gettler aptly coined the term "breastsleeping" to emphasize the very close relationship between bed-sharing and breastfeeding.

Adapted from Blair et al. (2010)
Breastfeeding frequencies over time for 4 different categories of bed-sharing
Source: Adapted from Blair et al. (2010)

The oft-stated conclusion by McKenna and colleagues that bed-sharing is not harmful and actually has several benefits has just received crucial support from a carefully designed assessment of the risk of SIDS by Peter Blair and colleagues. The crucial finding is that risk associated with bed-sharing was not significantly higher for infants less than 3 months old and was actually beneficial for older infants, provided that parents with hazardous behaviour were excluded (co-sleeping on a sofa, excessive drinking, smoking or taking drugs). This finding resolves a paradox inherent in past claims that bed-sharing is associated with a higher incidence of SIDS: Bed-sharing promotes breastfeeding, which is correlated with a lower incidence of SIDS, so surely we should expect that SIDS would be less likely with bed-sharing?

Proceed with Caution

In sum, appropriate bed-sharing by parents who do not smoke, drink heavily or take drugs does not increase the risk of infant death but does promote breastfeeding. Regardless of whether an infant sleeps in a separate room or in the same room as the parents, in their bed or in a separate crib, these guidelines apply: An infant should always sleep on its back on a firm, clean surface (in a bed, not on a sofa), under a light blanket that should never cover the head. That is the main take-home message. Another is that we also need to think about fathers sleeping with babies, but that is a topic that has yet to be properly researched.


Anderson, J.R. (2000) Sleep-related behavioural adaptations in free-ranging anthropoid primates. Sleep Medicine Reviews 4:355-373.

Blair, P.S., Heron, J. & Fleming, P.J. (2010) Relationship between bed sharing and breastfeeding: Longitudinal, population-based analysis. Pediatrics 127 e1119–e1126.

Blair, P.S., Sidebotham, P., Pease, A. & Fleming, P.J. (2014) Bed-sharing in the absence of hazardous circumstances: Is there a risk of Sudden Infant Death Syndrome? An analysis from two case-control studies conducted in the UK. PLoS ONE 9(9):e107799:1-7.

Carpenter, R., McGarvey, C., Mitchell, E.A., Tappin, D.M., Vennemann, M.M., Smuk, M. & Carpenter, J.R. (2013) Bed sharing when parents do not smoke: is there a risk of SIDS? An individual level analysis of five major case-control studies. BMJ Open 3(5): e002299.

Gettler, L.T. & McKenna, J.J. (2011) Evolutionary perspectives on mother-infant sleep proximity and breastfeeding in a laboratory setting. American Journal of Physical Anthropology 144:454-462.

Huang, Y., Hauck, F.R., Signore, C., Yu, A., Raju, T.N.K., Huang, T.T.-K. & Fein, S.B. (2013) Influence of bedsharing activity on breastfeeding duration among US mothers. JAMA Pediatrics 167:1038-1044.

Martin, R.D. (2013) How We Do It: The Evolution of Sex, Childbirth and Parenting. New York: Basic Books.

McKenna, J.J. (2007) Sleeping with Your Baby: A Parent's Guide to Cosleeping. Washington, DC: Platypus Media.

McKenna, J.J., Ball, H.L. & Gettler, L.T. (2007) Mother-infant cosleeping, breastfeeding and sudden infant death syndrome: What biological anthropology has discovered about normal infant sleep and pediatric sleep medicine. Yearbook of Physical Anthropology 50:133-161.

McKenna, J.J. & Gettler, L.T. (2015) There is no such thing as infant sleep, there is no such thing as breastfeeding, there is only breastsleeping. Acta Paediatrica doi: 10.1111/apa.13161.

Moon, R.Y. & Fu, L. (2012) Sudden infant death syndrome: an update. Pediatrics in Review 33:314-320.

Mother-Baby Behavioral Sleep Laboratory website: http://cosleeping.nd.edu/

Task Force on Sudden Infant Death Syndrome (2011) SIDS and other sleep-related infant deaths: expansion of recommendations for a safe infant sleeping environment. Pediatrics 128:1030-1039.

Vennemann, M.M., Bajanowski, T., Brinkmann, B., Jorch, G., Yücesan, K., Sauerland, C., Mitchell, E.A. & GeSID Study Group (2009) Does breastfeeding reduce the risk of sudden infant death syndrome? Pediatrics 123:e406-410.

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