The Science of Sharing Sleep

For species such as primates,
the mother is the environment.

~ Sarah Blaffer Hrdy (1)

by Lee T. Gettler and James J. McKenna

Although every human female is different, there is no doubt that her body is endowed with a unique capacity to breastfeed, should she choose to do so. The human infant is likewise biologically designed to sleep next to his or her mother’s body and to breastfeed intermittently throughout the night, at least for the first few years of life. In fact, nothing that a human neonate does makes sense except in light of the mother’s body. (2, 3)

Why is mother-baby contact important both day and night?

Although infant sleeping environments vary enormously from culture to culture, the potentially beneficial regulatory and developmental effects of contact on infants do not. Whether born in Brazil, Sweden, the US, the UK, or Nepal, whether living in a hunting-gathering society or an industrialized city: When resting on their mothers’ torsos, both premature and full-term infants breathe more regularly, use energy more efficiently, maintain lower blood pressure, grow faster, and experience less stress. (4–8) These data suggest that sensory exchanges with the mother alter and potentially regulate the immature physiology of the human infant—a primate mammal who starts life with only 25% of its adult brain volume, making it one of the least neurologically mature mammals at birth. Nighttime mother-infant proximity in the sleep environment likewise facilitates a variety of positive bio-behavioral experiences for the infant as well as for the mother herself.

In addition to its effect on the infant’s body temperature, brain-cell connections, calorie absorption, breathing, sleep, arousal patterns, and heart rate, proximity and contact are crucial for optimal breastfeeding. Hence, it is not surprising to find that cosleeping and breastfeeding represent a highly integrated and functionally interdependent system. Among exclusively breastfeeding mothers, the choice to cosleep, specifically in the form of mother infant bedsharing, was found to create a cascade of related changes in terms of both the behavior and the physiology of mother and infant. (9)

Most relevant to our concerns in this article, McKenna and colleagues documented a significant increase not only in the number of breastfeedings, but also in the total nightly durations of breastfeeding in the bedsharing environment, compared to when babies slept alone. (10) Different laboratories have recorded different total nightly durations of breastfeeding in the bedsharing-breastfeeding dyad, but all have found that, when a baby sleeps next to mother, the number of breastfeeding sessions per night increases significantly. (11–13)

Bedsharing also correlated with shorter average intervals between breastfeeding sessions. Among 70 nearly exclusively breastfeeding Latina mothers, McKenna’s team found that, when bedsharing, the average interval between breastfeedings was approximately 90 minutes. When sleeping in separate bedrooms (but still within earshot), the interval was at least twice as long. (14)

The increase in frequency and duration of breastfeeding associated with bedsharing has many benefits for mother and infant alike. For the infant, some of these benefits include better immunological protection during early infancy, provided by maternal antibodies present in breastmilk; proper development of the immunological and digestive systems via exposure to maternal biological agents; and delivery of the evolved package of nutritional support, in precise quantities, crucial to fueling rapid early natal body and brain growth.

Mothers who breastfeed frequently throughout the 24 hours of the day are more likely to experience lactational amenorrhea,15 which may reduce the risk of certain female reproductive cancers; (16) are better able to initiate and maintain breastfeeding; (17) and are frequently found to associate positive socio-emotional feelings with their breastfeeding/contact experiences with their infant, enhancing the mother’s validation as one whose presence is obviously appreciated by the positive changes exhibited by her infant. (18) All of these maternal benefits are increased or modulated as a consequence of the hormonal surges of prolactin and oxytocin associated with frequent nipple stimulation and suckling by the infant. (19) These hormones are known to be important for the onset and maintenance of maternal behavior in nonhuman mammals, and, on a behavioral level, may play a role in facilitating or enhancing (not necessarily causing) the positive feelings associated with maternal experiences, especially during breastfeeding. (20)

A common misperception of and concern about bedsharing in the US is that parents choosing to bedshare will habituate to the presence of their infant and be more prone to overlaying the infant. McKenna’s work demonstrated unambiguously that the opposite is the case, documenting that mothers who routinely bedshare exhibit an acute sensitivity to an infant’s presence in the bed. (21) These mothers awoke significantly more often during the bedsharing night in the laboratory than did routinely solitary-sleeping mothers on their bedsharing night. This finding argues against the likelihood that bedsharing mothers who consciously choose to bedshare under safe conditions (i.e., it is not obligatory, as in some impoverished circumstances; or accidental, such as on a sofa) habituate to the presence of their babies and thus may pose a danger of overlaying them while asleep. (22) It is important to delineate differences between the active choice to bedshare safely and circumstances in which the behavior is practiced without proper precaution (see "Safe Bedsharing" at end of this article).

The leap of logic that frequently springs from the revelation that bedsharing moms wake more often is, “Oh, so bedsharing means less sleep for mothers.” Surprisingly, even though they woke more often and fed their infants more frequently, routinely bedsharing mothers enjoyed as much sleep as breastfeeding mothers who routinely slept alone. (23)

When you consider the opportunities for feeding and comforting granted by the different sleeping arrangements, perhaps this is not so surprising. Under most circumstances, all a bedsharing mom has to do to allow a hungry infant access to her breast is to open her nightgown, or offer tender touches and reassuring kisses to an infant who is already within arm’s reach.

A solitary-sleeping mother, on the other hand, must vacate the comforts of her own bed to retrieve her infant, feed or comfort him, then return to bed and attempt to get to sleep, perhaps after a half hour or more of being awake. Thus, while bedsharing mothers wake more often, it is our experience that their arousals are shorter and less disruptive of sleep than those of solitary-sleeping moms. In light of this, it is not difficult to imagine why 94% of the routinely bedsharing mothers evaluated their sleep following their bedsharing night in the laboratory as enough, compared with 80% of the routinely solitary-sleeping mothers, following their normal sleeping arrangement of sleeping alone. (24)

Although many families may have no intention of bedsharing before the birth of their child, most parents end up practicing it, with varying frequencies and durations, as a means of facilitating nighttime feeding. This has become especially true as US breastfeeding rates have rebounded from their all-time lows in the 1970s, and as the incidence of bedsharing in the US has likewise risen. For instance, in one survey, 84% of routinely bedsharing mothers responded, before their infants were born, that they had no intention of bedsharing. (25)

Similarly, in one of her early studies in Great Britain, Dr. Helen Ball and colleagues contacted 60 mothers in prenatal interviews regarding their intentions for childcare practices. Forty of these mothers were then interviewed regarding their actual childcare practices two to four months after the birth of their infants. At this follow-up, it was found that 70% of new parents bedshared at least occasionally, despite the fact that 0% had intended to at the time of their prenatal interviews.

Furthermore, 35% of experienced parents anticipated bedsharing, whereas 59% were actually doing so at the time of follow-up. (26) As is probably the case for most families, especially those engaging in breastfeeding, the motivation to bring one’s baby into bed is often strong and logical, even if unplanned or unexpected, as it eases the transition from sleeping to feeding and back again.

While it remains speculative to say that solitary infant sleep is stressful for human infants, a variety of researchers have demonstrated that short-term separations of nonhuman primate infants from their mothers leads to an array of potentially life-threatening physiological changes such as adrenal-cortisol surges, immune dysfunction, and breathing abnormalities. Furthermore, leaving nonhuman primate infants alone to sleep induces serious impairments to sleep architecture, cardiac arrhythmias, and a variety of depressive syndromes. (27-30)

We know very little about the extent to which human infants experience stress when left completely alone for extended periods of time, as it is difficult to get ethical approval for such a research design, which represents a potentially dangerous, traumatic, or otherwise unsettling event for infants. However, Dr. Megan Gunnar and colleagues have shown that when nine-month-old infants are left in a room with a relatively inattentive adult, they indeed experience a physiological stress response similar in pattern (though the magnitude of the response is not as robust) to that experienced by nonhuman primate infants. (31)

It remains a curiosity of western cultures that we are largely unable to study prolonged mother-infant separation in a laboratory setting, even though our culture in the US provides us the perfect “natural experiment,” with thousands of neonates and infants being left to sleep alone for long periods of time every night. However, it is reasonable to at least suggest that solitary infant sleep represents an evolutionarily anomalous setting for human babies. As a result, infants may experience a physiological stress response that puts them at risk of energy depletion and immunological depression, while placing undue strain on their developmentally immature respiratory, cardiac, and neurological systems.

History tells us that solitary infant sleep does not pose a risk of death or severe developmental insult to most western infants, and many millions of westerners have gone on to healthy, happy lives after being left to sleep alone as infants. However, exposure to repeated stressors could prove particularly deleterious for infants with genetic or developmental neurological deficiencies, such as those believed to be involved with Sudden Infant Death Syndrome (SIDS), and could allow such deficits to find expression when they may otherwise have been averted by the regulation provided by the mother’s body and her watchful eye in the safe cosleeping environment.

Bedsharing is just one of many forms of cosleeping, and while all bedsharing represents a more intimate type of cosleeping in which the caregiver and infant share a sleeping surface, not all cosleeping takes the form of bedsharing. Moreover, safe bedsharing (see "Safe Bedsharing" at end of this article) can now be distinguished from unsafe bedsharing. (32) For these reasons, the terms cosleeping and bedsharing are not synonymous and should not be used interchangeably—a distinction not acknowledged by scholarly condemnations of “cosleeping” and “bedsharing.” (33, 34)

Furthermore, bedsharing risks or protective factors are best conceptualized as occurring along a benefits-risk continuum. For example, when highly committed, nonsmoking, breastfeeding mothers elect to bedshare for nurturing purposes, positive outcomes can be expected. On the other hand, when the physical environment for bedsharing is less than optimal, negative outcomes can occur. Suboptimal circumstances for bedsharing include smoking, bottle feeding, sharing the bed with inattentive or unaware individuals, and abusing drugs or alcohol. Other unsafe sleeping situations include sleeping with infants on couches or recliners rather than beds. Outcomes also tend to be less positive among mothers who bedshare out of necessity because they cannot afford a crib. (35, 36)

Roomsharing as a Form of Cosleeping that Helps Protect Infants from SIDS

While recommending against bedsharing to reduce the chances of SIDS, the American Academy of Pediatrics (AAP) enthusiastically supports and recommends another form of cosleeping called roomsharing. In this situation, the committed caregiver and infant sleep close enough together for sensory exchanges, though not on the same surface.

As defined in the scientific literature, (37) safe cosleeping refers to any sleeping arrangement in which a sober, committed caregiver and infant sleep close enough for each to detect, exchange, and respond to the other’s sensory signals and cues, whether sleeping on the same surface or not. It seems silly to have to point this out, but the AAP seems reluctant to acknowledge that it is not the room that protects the baby, but the mother (or father) in the room, and what they do for and to their infant while cosleeping, who do the protecting. That said, this is the first time that any prestigious western medical organization has stated that a mother’s presence or proximity can be critical to the survival of her infant—and that infants should never sleep alone. Epidemiological data show that, in the presence of an adult caregiver, roomsharing infants are approximately half as likely to die of SIDS than infants sleeping either alone or in the same room with siblings. (38–41)

Indeed, these findings also show that it takes a committed adult caregiver to achieve these protective effects, as the findings did not generalize to the presence of other children in the infant’s room. As has been argued elsewhere, (42) this provides evidence that a mother’s presence plays a proactive, protective role, putting her in the position to detect and respond to deleterious changes in her infant’s status, while simultaneously inducing biological changes through her sensory exchanges that may help override inherent neurobiological deficits that increase the likelihood of SIDS. (43–45)

Recent public health campaigns in the US have included the following messages to parents: “Babies sleep safest alone” (New York State); “For you to rest easy, your baby must rest alone,” (46) and “All babies should be placed to sleep in cribs” (Philadelphia). These public health efforts represent drastic departures from the AAP recommendation in favor of roomsharing, which emphasizes the importance of parent-child proximity.

The Philadelphia campaign specifically references the AAP recommendation, but with no mention that the AAP recommends that babies never sleep alone, and always near an adult caregiver. While the New York campaign is at least consistent with the AAP’s message in favor of roomsharing, the unqualified public announcement that “Babies sleep safest alone,” as disseminated through TV and radio advertisements, makes no such acknowledgment of the importance of caregiver-infant nighttime proximity. (47) This unequivocally gives the impression that what is best for infants is to sleep alone in a crib separated from caregivers altogether.

In both of the aforementioned campaigns, public health officials are trying to reduce the number of deaths due to unsafe bedsharing practices, which they erroneously imply are representative of not only all bedsharing practices, but of all cosleeping environments as well. In oversimplifying a fundamental act of human affectionate behavior and biology, they do more harm than good by delivering messages that are easily interpreted as imploring parents to leave their babies to sleep entirely separated from caregivers, against the recommendation of the AAP, and thus increasing the likelihood of SIDS.

But only when sweeping public health recommendations acknowledge and respect maternal capacities, as well as the biologically appropriate emotions and motivations of mothers to sleep close to their infants, will there be any hope that these recommendations will be adopted and implemented in ways that will promote the survival and well-being of the greatest numbers of mothers and infants.

Lee T. Gettler is Associate Director of the Mother-Baby Behavioral Sleep Lab, University of Notre Dame, Department of Anthropology; and a doctoral student at Northwestern University’s Department of Anthropology.

James J. McKenna, PhD, is Director of the Mother-Baby Behavioral Sleep Lab, and holds the Rev. Edmund P. Joyce C.S.C. Chair in Anthropology, at the University of Notre Dame, Department of Anthropology.

Safe Bedsharing Tips

• If bottle-feeding (without breastfeeding), or if mother smoked during pregnancy, practice side-by-side, separate-surface cosleeping using a crib, bassinet, or an Arm’s Reach Co-Sleeper.

• If routinely bedsharing, it is best to strip the bed frame from the bed and place mattress and box springs in center of room, away from all walls unless they are heavy and secure enough on the floor to NOT pull away from the walls. Mattresses pushed against walls can pull away, leaving dangerous spaces into which babies can become wedged and suffocate.

• If the bed frame is present, eliminate any spaces or gaps between mattress and head or footboard, and keep bed away from adjacent furniture, which can create spaces into which a baby can fall and suffocate.

• No children should sleep in an adult bed with an infant.

• Families should avoid bedsharing when overly exhausted, desensitized by drugs or alcohol, or sleeping with an unrelated adult.

• Bedshare only on stiff mattresses, always lay baby on his or her back, avoid using duvets or heavy blankets, and keep infant away from pillows, or anything that obstructs airflow around infant’s face.

• Never leave infant alone on an adult bed, never cosleep on a couch, sofa, recliner, or chair, and never bedshare on a waterbed.

• If bedsharing includes two adults, both should agree to be responsive to and vigilant for infant.

For a list of safe cosleeping tips, see:
The University of Notre Dame’s Mother-Baby Behavioral Laboratory and The website of Sarah J. Buckley, MD

More information, links, and research articles on sharing sleep here.


1. Sarah Blaffer Hrdy, Mother Nature: Maternal Instincts and How They Shape the Human Species (New York: Pantheon Press, 1999).
2. Ibid.
3. J. J. McKenna, H. L. Ball, and L. T. Gettler, “Mother-Infant Co-Sleeping, Breastfeeding and Sudden Infant Death Syndrome (SIDS ): What Biological Anthropology Has Discovered about Normal Infant Sleep and Pediatric Sleep Medicine,” Yearbook of Physical Anthropology 50 (2007): 133–161.
4. G. C. Anderson, “Current Knowledge about Skin-to-Skin (Kangaroo) Care for Preterm Infants,” Journal of Perinatology 11, no. 3 (September 1991): 216–226.
5. S. M. Ludington, “Energy Conservation During Skin-to-Skin Contact Between Premature Infants and their Mothers,” Heart Lung 19 (September 1990): 445–451.
6. S. M. Ludington-Hoe, A. J. Hadeed, and G. C. Anderson, “Physiologic Responses to Skin-to-Skin Contact in Hospitalized Premature Infants,” Journal of Perinatology 11, no. 1 (March 1991): 19–24.
7. S. M. Ludington-Hoe, A. J. Hadeed, and G. C. Anderson, “Randomized Trials of Cardiorespiratory, Thermal and State Effects of Kangaroo Care for Preterm Infants,” Society for Research in Child Development Biennial Meeting (Seattle, WA: 19 April 1991).
8. S. M. Ludington-Hoe et al., “Selected Physiologic Measures and Behavior During Paternal Skin Contact with Colombian Preterm Infants,” Journal of Developmental Physiology 18, no. 5 (November 1992): 223–232.
9. J. J. McKenna, “Cultural Influences on Infant and Childhood Sleep Biology and the Science that Studies It: Toward a More Inclusive Paradigm.” In: J. Loughlin et al., eds., Sleep and Breathing in Children: A Developmental Approach (New York: Marcel Dekker, 2000), 99–130.
10. J. J. McKenna, S. Mosko, and C. Richard, “Bedsharing Promotes Breastfeeding,” Pediatrics 100 (August 1997): 214–219.
11. H. L. Ball, “Breastfeeding, Bed-Sharing, and Infant Sleep,” Birth 30, no. 3 (26 August 2003): 181–188.
12. S. A. Baddock et al., “Sleep Arrangements and Behavior of Bed-Sharing Families in the Home Setting,” Pediatrics 119, no. 1 (January 2007): e200–e207.
13. J. Young, “Night-Time Behavior and Interactions Between Mothers and their Infants of Low Risk for SIDS : A Longitudinal Study of Room Sharing and Bed Sharing,” unpublished doctoral thesis, University of Bristol (1999).
14. See Note 10.
15. P. T. Ellison et al., “The Ecological Context of Human Ovarian-Function,” Human Reproduction 8, no. 12 (December 1993): 2248–2258.
16. J. L. Kelsey, M. D. Gammom, and E. M. John, “Reproductive Factors and Breast Cancer,” Epidemiologic Reviews 15, no. 1 (1993): 36–47.
17. H. L. Ball et al., “Randomised Trial of Infant Sleep Location on the Postnatal Ward: Implications for Breastfeeding Initiation and Infant Safety,” Archives of Disease in Childhood 91 (December 2006): 1005–1010.
18. R. Rigda et al., “Bed Sharing Patterns in a Cohort of Australian Infants During the First Six Months After Birth,” Journal of Paediatrics and Child Health 36, no. 2 (2000): 117–121.
19. R. A. Lawrence and R. M. Lawrence, Breastfeeding: A Guide for the Medical Profession, sixth ed. (Orlando: Mosby Inc., 2005).
20. K. Uvnäs-Moberg and D. Magnusson, “The Psychobiology of Emotion: The Role of the Oxytocinergic System,” International Journal of Behavioral Medicine 12, no. 2 (June 2005): 59–65.
21. S. Mosko, C. Richard, and J. J. McKenna, “Maternal Sleep and Arousals During Bedsharing with Infants,” Sleep 20, no. 2 (1997): 142–150.
22. Ibid.
23. Ibid.
24. Ibid.
25. J. J. McKenna and L. E. Volpe, “Sleeping with Baby: An Internet-Based Sampling of Parental Experiences, Choices, Perceptions, and Interpretations in a Western Industrialized Context,” Infant and Child Development 16, no. 4 (28 August 2007): 359–385.
26. H. L. Ball, E. Hooker, and P. J. Kelly, “Where Will the Baby Sleep? Attitudes and Practices of New and Experienced Parents Regarding Cosleeping with their Newborn Infants,” American Anthropologist 10, no. 1 (1999): 143–151.
27. C. L. Coe, “Psychosocial Factors and Immunity in Nonhuman-Primates: A Review,” Psychosomatic Medicine 55, no. 3 (1993): 298–308.
28. M. Laudenslager et al., “Possible Effects of Early Separation Experiences on Subsequent Immune Function in Adult Macaque Monkeys,” American Journal of Psychiatry 142, no. 7 (1985): 862–864.
29. M. L. Laudenslager et al., “Behavioral and Immunological Consequences of Brief Mother-Infant Separation: A Species Comparison,” Developmental Psychobiology 23, no. 3 (1990): 247–264.
30. M. Reite et al., “Maternal Separation in Bonnet Monkey Infants: Altered Attachment and Social Support,” Child Development 60, no. 2 (April 1989): 473–480.
31. M. R. Gunnar et al., “The Stressfulness of Separation Among 9-Month- Old Infants: Effects of Social-context Variables and Infant Temperament,” Child Development 63, no. 2 (1992): 290–303.
32. J. J. McKenna and S. Mosko, “Mother-Infant Cosleeping: Toward a New Scientific Beginning.” In: R. Byard and H. Krous, eds., Sudden Infant Death Syndrome: Problems, Puzzles, Possibilities (New York: Arnold Publishing, 2001).
33. D. A. Drago and A. L. Dannenberg, “Infant Mechanical Suffocation Deaths in the United States, 1980–1997,” Pediatrics 103, no. 5 (1999): e59.
34. S. Nakamura, M. Wind, and M. D. Danello, “Review of Hazards Associated with Children Placed in Adult Beds,” Archives of Pediatrics & Adolescent Medicine 153 (1999): 1018–1023.
35. J. J. McKenna and T. McDade, “Why Babies Should Never Sleep Alone: A Review of the Co-Sleeping Controversy in Relation to SIDS , Bedsharing and Breastfeeding,” Paediatric Respiratory Reviews 6 (2005): 134–152.
36. See Note 32.
37. J. J. McKenna et al., “Infant-Parent Co-Sleeping in Evolutionary Perspective: Implications for Understanding Infant Sleep Development and the Sudden Infant Death Syndrome (SIDS ),” Sleep 16 (1993): 263–282.
38. R. G. Carpenter et al., “Sudden Unexplained Infant Death in 20 Regions in Europe: Case Control Study,” The Lancet 363, no. 9404 (2004): 185–191.
39. E. A. Mitchell and J. M. D. Thompson, “Cosleeping Increases the Risks of Sudden Infant Death Syndrome, But Sleeping in the Parent’s Bedroom Lowers It.” In: T. O. Rognum, Sudden Infant Death Syndrome in the Nineties (Oslo: Scandinavian University Press, 1995), 266–269.
40. P. S. Blair et al., “Where Should Babies Sleep—Alone or with Parents? Factors Influencing the Risk of SIDS in the CESD I Study,” British Medical Journal 319 (1999): 1457–1462.
41. P. Fleming et al., “Environments of Infants During Sleep and Risk of Sudden Infant Death Syndrome: Results of 1993–1995 Case Control Study for Confidential Inquiry into Stillbirths and Deaths in Infancy,” British Medical Journal 313 (1996): 191–195.
42. See Note 3.
43. See Note 37.
44. S. Mosko et al., “Parent-Infant Co-Sleeping: The Appropriate Context for the Study of Infant Sleep and Implications for SIDS Research,” Journal of Behavioral Medicine 16, no. 3 (1993): 589–610.
45. S. Mosko et al., “Infant Sleep Architecture During Bedsharing and Possible Implications for SIDS ,” Sleep 19 (1996): 677–684.


  1. Love the article.. Thank you for making my breastfeeding feel more normal and appreciated. xx

  2. Excellent article! It is funny how research always seems to back up a mother's natural instincts... if only more parents followed their instincts and ignored the bad advice (that is constantly changing).

  3. Great article ... follow your instinct to keep breastfeeing into toddlerhood and keep your baby close to you! Reject the nonsense that forces you to parent at arms length!



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