The effects of bed sharing on sleep: from partners to pets

Abstract

Bed sharing is common practice across the global population. However, the vast majority of research on bed sharing has focused solely on mother-infant bed sharing. Here we provide a holistic review of research on bed sharing. Articles investigating the relationship between bed sharing and sleep were identified in four dyad categories: (1) parent and child, (2) couples, (3) siblings, and (4) pet owners and pets. Of interest was whether sleep-promoting factors such as psychological comfort were generalizable across bed-sharing dyads; alternatively, sleep-demoting factors such as movement or heat may be commonalities. We found that, across dyad types, in general, subjective reports of sleep quality were better when bed sharing despite generally worse objective measures of sleep. Understanding bed sharing is important to treating sleep disturbances, given the prevalence of shared beds. This scoping review points to critical gaps in our understanding of bed sharing that motivate future research.

Introduction

Although sleep represents a series of brain states that allow little to no responsiveness to the people around us, sleep is commonly a social experience. For 82% of married, partnered, or coliving couples in the United States, sharing a bed is routine ¹. From the same poll, 32% report bed sharing with a child or pet.

Bed sharing is defined as a person sharing a single mattress with other people or animals. Although the term “co-sleeping” is often used to refer to bed sharing, “co-sleeping” is also used to refer to room sharing. While bed sharing and room sharing may have similar influences on sleep, the proximity of the individuals when bed sharing is likely to enhance the influences of co-sleeping, whether positive or negative.

Here we review how bed sharing affects sleep. The intent of this scoping review is to summarize what is currently known about primarily healthy individuals’ sleep and to examine whether there are commonalities across bed sharing situations and factors that should be considered in future experimental research and systematic reviews. Bed sharing is a common practice, and the positive or negative effects of bed sharing on sleep may provide insight into ways to improve sleep from a wide population. Additionally, it is important to consider the importance of bed sharing since most clinical and research sleep studies are done in the laboratory without bed sharing, which may produce different results than a subject’s normal sleep environment.

A brief history of sleeping arrangements

During the early Middle Ages, sleep was not considered a private affair. In medieval England, beds were designed to be communal ². Bed sharing was considered both normal and practical – necessary for security, conservation of resources, and generation of heat ^3,4.

Most historical research on bed sharing is specific to spouses. The sociocultural view of sleep as a solitary activity did not come to the forefront until the late Victorian Era (1837-1901). Ideas about health became increasingly tied to cleanliness and, in an effort to address public health issues, people began to strictly follow hygienic measures. Victorian sensibilities as to what was healthy lent focus to bed-sharing practices. Gradually, medical beliefs of this era led to the view that bed sharing was a vehicle for disease transmission, which inspired the adoption of the twin bed ⁵.

The twin bed grew in popularity because it was regarded as more hygienic than the double bed ⁵. Preceding germ theory, which was only beginning to be uncovered by Pasteur’s experiments starting in 1800-1864, the leading scientific explanation for the origin and spread of disease was the miasma theory. Miasma theory held that diseases spread through poisonous particles in the air and that these particles could be identified by foul odors. Under this theory, a person could fall ill by inhaling any foul odor ⁶. A partner’s ‘morning breath’ was seen as a health risk in the context of miasma theory. Twin beds provided a way to reduce exposure to foul odors emanating from bed partners and thus were believed to reduce health risks.

In addition to the risk of disease, bed partners were thought to deplete one’s ‘vital forces,’ or energy after a night spent bed sharing. It was often observed that one bed partner would fall asleep easily and wake up feeling well rested, while the other would find difficulty in sleeping and would arise in the morning feeling more tired than when they fell asleep ⁵. This differential effect between bedmates was explained by the popular belief that two individuals sharing a bed took on the role of nervous energy absorber or nervous energy eliminator. The absorber would absorb the energy from the eliminator during the night, leaving the eliminator feeling unrested. The effects of bed sharing between people of differing energies soon became understood as unhealthy for general well-being, for demeanor, and for efficiency at work.

Additionally, Victorians favored privacy. This view led to wealthier individuals of the time having separate bedrooms for the matriarch and patriarch of the house, while those in the lower class who could not afford to have separate rooms instead had separate twin beds within the same room. While husband and wife began to sleep in separate rooms or beds, children were also moved out of their parents’ bedroom and into their own space ⁷.

Parent-child bed sharing has likewise been historically prevalent and has changed with medical and cultural views. Mother-infant bed sharing has, at times, been blamed for infant deaths, resulting in an outright ban on mothers sharing a bed with any child under 4 years in some countries in the 1200s ⁸. A similar view was also reflected in a medical report in 1862, as English physician Anthony Leared wrote of several deaths of infants who died of apparent suffocation by their mothers or nurses.

Although seldom singled out in the literature, siblings commonly shared a bed with each other and their entire family. When sleep was outdoors or in basic structures, family bed sharing provided safety and warmth. Once modern beds in homes were available, beds were expensive and thus shared by the family for economic reasons ⁷. While miasma theory moved couples into their own beds in the 19^th century, bed sharing often continued amongst young children since miasma was thought to be a symptom of age. Nonetheless, in 1800, a single bed was recommended for children for comfort and health ⁹.

What we know of the history of bed sharing with animals is also scant. Drawing from studies of non-industrialized cultures, it is presumed that early bed sharing with animals may have also been for the purpose of warmth and safety ¹⁰. The term ‘three dog night’, credited to Aboriginal culture, refers to a night so cold three dogs were needed in bed for sufficient heat ¹¹. However, there is anecdotal evidence of bed sharing with animals from a variety of cultures throughout history, for psychological benefits as well as more practical ones. Examples of high-profile individuals who slept alongside their pets include Alexander the Great, who slept next to his mastiff after battles, ¹⁰ and Mary Queen of Scots, who slept with her spaniels while imprisoned in the Tower of London awaiting her execution ¹².

While the aspects of sleep that have been covered thus far reflect the past, these changes have created =social norms that have an impact on the way that co-sleeping is viewed today in Western civilization. Moving forward, it is important to consider consequences of bed sharing on sleep. We focus on four dyad types for which reports of sleep are available: (1) parent and child, (2) couples, (3) siblings, and (4) pet owners and pets. In the following sections, we review what is known about bed sharing, with particular interest in commonalities across dyad types.

Bed sharing in couples

Laboratory studies suggest that sleep architecture may differ for bed sharing couples and non-bed sharing couples. When bed sharing, both bed partners were found to have increased REM sleep compared to when sleeping alone ^13,14. However, other effects are less consistent across studies of typically-functioning healthy adults, possibly because of the limited number of studies and the small sample sizes within these studies (Table 1). For instance, one early study conducted by Monroe ¹³ used polysomnography to show that slow wave sleep was greater and night awakenings were fewer when sleeping alone. In contrast, a more recent study conducted by Drews and colleagues ¹⁴ used polysomnography and found both less sleep and less sleep efficiency when sleeping alone compared to bed sharing.

Table 1.

Experiments examining sleep quality in bed sharing couples

Authors	Study Population	Sleep Quality Measurement	Main Findings
Diamond, Hicks, & Otter-Henderson (2008)	42 heterosexual couples	Measures of salivary cortisol and daily sleep diaries over a 21-day period with naturally occurring separations of four to seven days	Both the partner who stayed at home and the partner who was traveling reported greater sleeping problems when apart. Both partners reported significantly better sleep after the separation when they once again bed shared.
Dittami, Keckeis, Machatschke, Katina, Zeitlhofer, & Kloesch (2007)	10 heterosexual couples	Actigraph and sleep log over 20 nights (10 bed sharing, 10 solitary sleeping)	Sharing a bed had negative effects on sleep in women. Sexual contact mitigated the negative subjective report, but objective measures were unaffected. Subjective assessment of sleep quality was lower with men than women when sleeping alone; they increased to the same level as women while sleeping in pairs. Sleep efficiency in men was not reduced by bed sharing unless sexual contact occurred.
Drews, Wallot, Weinhold, Mitkidis, Baier, Roepstorff, & Goder (2017)	4 heterosexual couples	PSG over four nights (two nights bed sharing, two nights solitary sleeping)	Bed sharing was associated with better subjective sleep quality, increased total sleep time, sleep efficiency, total slow wave sleep, and REM sleep. Sleep stages were more synchronized when bed sharing independent of awakenings.
Monroe (1969)	14 heterosexual married couples	PSG measured with the couples sleeping three nights as a pair and independently.	Couples experienced significant decrease in stage 4 nREM sleep and a significant increase in REM sleep and awakenings when sleeping together as opposed to sleeping alone. Women had a larger decrease in stage 4 nREM sleep than their male partners, though this difference did not reach significance.
Pankhurst & Horne (1994)	46 heterosexual couples and 39 solitary sleepers	Actigraph and morning sleep diaries over eight consecutive nights	There was a substantial amount of movement concordance amongst bed sharers. Subjects who habitually shared a bed with their partner had longer sleep duration than those who slept alone. Most subjects reported that they had better sleep with a partner than alone and seemed unaware of possible detriments to sleep quality as a result of sleeping with a partner.
Spiegelhalder, Regen, Siemon, Kyle, Feige, Nissen, & Reimann (2017)	15 heterosexual couples	Actigraph and sleep diaries over five consecutive nights; and sleep questionnaires: PSQI, ISI, PSAS, DBAS-16, GSES, the ESS	Men slept longer and got out of bed later when bed sharing. Sleep quality was believed to be better by both sexes when co-sleeping. Higher concordance of partners’ body movements when sleeping together as opposed to sleeping apart.

PSQI, Pittsburgh sleep quality index (89); ISI;, Insomnia severity index (90), PSAS, Pre-sleep arousal scale (91); DBAS-16, Brief version of dysfunctional beliefs and attitudes about sleep (92); GSES, Glasgow sleep effort scale (93); ESS, Epworth Sleepiness Scale (94)

The difference in sleep time across studies possibly contributed to this difference, as one study had more sleep time on average (>6.5 hours) ¹³ than the other study (<6 hours) ¹⁴, which may account for greater time in slow wave sleep. Additionally, in the Monroe study¹³, couples slept in a lab for three consecutive nights, whereas the Drews study ¹⁴ had couples sleep in the lab for two consecutive nights twice. The design of the Drews study ¹⁴ reduces the effect of adaptation to sleeping with polysomnography and other sources of variability by increasing the number of nights spent in the lab. However, the findings of the more recent study may be countered by the fact that the Monroe study had a much larger sample (14 couples) ¹³ than the study conducted by Drews and colleagues (4 couples) ¹⁴.

Monroe ¹³ reported that among the couples studied, who were all heterosexual, females had more stage 4 nREM sleep (the ‘deepest’ form of slow wave sleep, although no longer distinguished from nREM3) and less REM when sleeping alone than with their partners. However, these differences were not large enough to reach significance. Conversely, a study utilizing actigraphy, sleep logs, and questionnaires found significant sex differences for total sleep time: males were found to sleep more when bed sharing, while sleep time for females did not differ when bed sharing compared to sleeping alone ¹⁵.

While males may get apparent benefits from bed sharing, female sleep may be disturbed by bed sharing with a spouse. For instance, females were reported to have less sleep fragmentation when sleeping alone compared to when bed sharing in a study that used sleep logs and actigraphy ¹⁶. Although the increase in awakenings for females is often explained by female social roles as caregivers ^17,18, such increased awakenings during bed sharing may also be explained by the fact that males are found to move more frequently while sleeping (determined through actigraphic measures ¹⁹), thus disturbing their female bed partner. Benefits of bed sharing observed in males have been explained by males being more likely to view closeness as an important social construct in a relationship ²⁰. Given these results, it is possible that same-sex couples could have less fragmented sleep than heterosexual couples. Two women sleeping together could increase sleep quality since women were found to move less frequently than men, decreasing sleep fragmentation of their partners. On the other hand, two men sleeping together could lead to increased sleep quality since they do not seem to wake as often as women. More research into same-sex couples and sleep differences will be an important next step.

Perhaps as a result of socially-constructed sleep beliefs, subjective measures suggest that couples often perceive their sleep to be better when bed sharing ^15,19. In fact, females report that their sleep is disturbed by their partner, yet still rated their sleep as better than when they slept alone¹⁹. One example of the psychological benefits of bed sharing is feeling more relaxed while bed sharing than while sleeping alone ¹⁴. In a series of qualitative interviews, many couples reported that they associate their partners’ presence in bed with feelings of comfort, warmth, security, and calm ²¹. One study utilizing sleep diaries reported that they have difficulty falling asleep if their partner is absent ²².

Although the described psychological benefits may support sleep quality, not all couples will experience the same magnitude of effects. Specifically, sleep quality is closely related to relationship quality ²³, and the relations between sleep quality and marital quality have been described as bidirectional¹⁸. For instance, if couples report higher sleep efficiency, they also tend to have fewer negative interactions with each other during the day. In turn, less negative interactions during the day predict high self-reported sleep efficiency the following night ²⁴. Additionally, high marital satisfaction among wives was associated with higher sleep concordance with their husbands, meaning that they were simultaneously sleeping for significantly more time than those with lower marital satisfaction ²⁵. This perception of sleep quality has been associated with responses on the secure attachment scale. Specifically, women who scored higher on the avoidant subscale reported lower sleep quality, showing that there are both physiological and psychological factors that lead to one’s opinion on their quality of sleep²⁶.

The subjective benefits of bed sharing with a partner seem to outweigh the negative effect of sleep fragmentation when sleeping with a partner. Whether these benefits appear because of societal expectations or a lack of understanding of the deficits that arise when sleep deprived is unclear. These motivations will continue to be of interest as we further explore the effects of bed sharing.

Bed sharing between parent and child

Sleeping arrangements are heavily influenced by cultural tradition ²⁷, which gives rise to distinct cross-cultural differences in parent-child bed sharing (Table 2). For example, a study of mothers in Korea reported that the prevalence of regular parent-child bed sharing was 88.2% among children 3 to 5 years ²⁸. Similarly, prevalence of parent-child bed sharing was 72% in Japan ²⁹ and 93% in India One study, which reviewed infant care practices across 10 hunter-gather societies and 176 non-industrialized societies, found mother-infant bed sharing in 100% of the hunter-gatherer societies and 76% of non-industrialized societies ³¹. However, only 9% of mothers in North America regularly bed share with their infants, and only 3% do so at age 6 years ³². Lower co-sleep rates in the U.S. may reflect recommendations from organizations such as the CDC, which dissuade parents from co-sleeping with their children based on safety concerns, including correlations between bed sharing and Sudden Infant Death Syndrome ^8,33–35,36. Although parent-child bed sharing is low overall in the United States, it is a more common practice in U.S. Black and Hispanic populations compared to non-Hispanic White populations ³⁷. Bed sharing is also associated with income, with low-income families and those who are not home owners being more likely to bed share ³⁸. Additionally, parents with higher occupational status were found to be more likely to have their infants (6-7 mos) sleep in a cot than to bed share.

Table 2.

Experiments examining sleep quality in parents bed sharing with their children

Authors	Study Population	Sleep Quality Measurement	Main Findings
Ball (2002)	253 mothers (120 mothers bed sharing in their infant’s first month, 74 mothers bed sharing in their infant’s third month)	Sleep diaries over a period of seven consecutive days during their baby’s first month and third month, interviews at the end of the first and third month	For the 19 percent of families where bed-sharing was unrelated to breast-feeding, settling a baby who was having trouble sleeping was a prevalent (55 percent) reason for bed-sharing, suggesting that parents thought bed sharing improved their infants’ sleep.
Chianese, Ploof, Trovato, & Chang (2009)	28 bed sharing caregivers	Four focus groups, dialogue was transcribed and coded to identify themes	All groups claimed that better infant and caregiver sleep was a large justification for bed sharing even with the possible health risks.
Cortesi, Giannotti, Sebastini, Vagnoni, & Marioni (2008)	148 children (ages five to nine years) with bedtime problems (44 bed sharers, 104 solitary sleepers) and 228 healthy peers	CSHQ and parent recorded sleep diaries	Bed sharers with bedtime problems had significantly higher scores on bedtime resistance, night awakenings, sleep duration, and sleep anxiety than solitary sleepers with bedtime problems.
Jiang, Chen, Spruyt, Sun, Wang, Li, Shen, Wang, & Jiang (2016)	1452 children (mean age 10.83 years), 244 were bed sharers	CSHQ	Bed sharers were more likely to have bedtime resistance, sleep anxiety, and poor sleep quality in comparison to non-bed sharers. Bed sharing was associated with daytime sleepiness but not with sleep duration.
Kim, Lee, & Cain (2017)	48 mothers (23 co-slept with their children)	CSHQ, PSQI	100 percent of the bed sharing children had sleep disturbances compared to 56 percent of the non-bed sharing children. 28 percent of the non-bed sharing mothers reported sleep disturbances, compared to 52 percent of the bed sharing mothers.
Mao, Burnham, Goodlin-Jones, Gaylor, & Anders (2004)	18 infants (9 routinely bed sharers, 9 solitary sleepers)	Recorded video of infants sleeping for two consecutive nights in their natural environment over at least two time points	Across age, bed sharing infants had more awakenings per night. The percent of the nighttime spent awake did not differ between groups, suggesting that bed sharing infants had shorter awakenings.
Mckenna & Volpe (2007)	200 bed sharing mothers	Questionnaire (nine questions related to bed sharing)	26 percent of mothers claimed that they endorsed bed sharing because it increased the quality of parental sleep.
Mosko, Richard, & McKenna (2002)	35 mother/infant pairs (20 routinely bed sharing and 15 routinely solitary sleeping)	Three consecutive nights of PSG (initial adaption night followed by co-sleeping and then solitary sleeping or vice versa) sleep diaries completed over 14 consecutive days	When bed sharing, mothers’ total sleep time was not decreased compared to the solitary night. Across the two groups, percent Stage 3–4 nREM sleep (of total sleep time) was significantly reduced on the bed sharing night, while Stage 1–2 nREM sleep was increased. The amount of REM sleep was unaffected. Overall, arousal frequency was significantly increased when bed sharing but nocturnal wakefulness did not because awakenings were of shorter duration.
Okami, Weisner, & Olmstead (2002)	205 parents (18 parents reported regularly sharing their beds with their child at one of the time points)	Sleep problems were assessed through parent interviews at the two- and six-year periods	No significant association between bed sharing and sleep problems.
Lee, Ha, Moon, Youn, Kim, Park, Kim, Kim & Chung	115 parents, 60 children (mean age 4 years)	CSHQ, Sleep environment survey, ISI, K-PSI-SF, PHQ-9	Children who shared a bed with their parents scored higher on the CSHQ. Path analysis showed that co sleeping can lead to sleep problems and insomnia symptoms in the mother.
Volkovitch, Zion, Karny, Meiri, Tikotzky (2015)	153 families (116 were room sharing or bed sharing at three and/or six months postpartum)	Actigraph and sleep diaries for five consecutive days at three months and six months postpartum	Co-sleeping and bed sharing did not differ and so were combined. Mothers of co-sleeping infants reported more infant night awakenings than mothers of solitary sleeping infants. None of the objective sleep measures were significantly different between co-sleeping and solitary sleeping infants after controlling for feeding techniques. Co-sleeping mothers had significantly more subjective sleep disturbances than mothers in the solitary sleeping group.

CSHQ, Child sleep health questionnaire (41); PSQI, Pittsburgh sleep quality index (89); Sleep environment survey (95); ISI, Insomnia severity index (96); K-PSI-SF, Korean parent stress index-Short form (97); PHQ-9, Patient health questionnaire (98)

While there are certainly environmental factors that may influence bed sharing, such as housing arrangements and economic access to beds ³⁹, there are two broad categories that define why parents choose to bed share with their children: reactive bed sharing and purposeful bed sharing ⁴⁰. In purposeful bed sharing, parents initiate and encourage the practice, often from birth. In reactive bed sharing, parents bed share in response to external factors, such as the child having nightmares and wishing to be comforted by their parents. Reactive bed sharing typically involves older children (5-9 years), who have been found to exhibit more bedtime resistance and sleep anxiety compared to non-bed sharing children in studies that utilized parent questionnaires ^41,42. These events are likely not direct effects of bed sharing, but rather are reasons that lead parents to reactively bed share in response to their child’s sleep problem.

In these cases, while bed sharing with older children is believed to be safe ⁴³, such practices may compromise the child’s sleep quality. Using the Children’s Sleep Habits Questionnaire ⁴⁴, a sleep screening survey designed for children aged 4 through 10 years, bed sharing children were found to have significantly shorter sleep duration and more behavioral and emotional problems than children who do not bed share ⁴¹. Additionally, in one study, parents filling out child sleep questionnaires found that bed sharing children experience more daytime sleepiness than non-bed sharers and tend to have more bed time resistance ⁴⁵. Parents of bed sharers also suffer, with mothers reporting higher rates of psychological distress and a higher likelihood of developing insomnia ⁴⁶, and both parents reporting that they experience higher levels of marital distress ⁴¹, although it should be noted that the directionality of such associations is not known. Nonetheless, although reactive bed sharing involving older children is intended to address the child’s sleep problems, it is possible that it impedes the child’s sleep quality and creates more sleep problems.

The practice of purposeful bed sharing has been met with resistance from U.S. health professionals, who discourage parent-infant bed sharing for the infant’s safety ⁴⁰, as bed sharing with infants is associated with increased likelihood of Sudden Infant Death Syndrome ⁴⁷. Other risks include accidental asphyxiation, entrapment, falls, overheating, and strangulation ^48–50. Parents typically initiate bed sharing because they believe it is beneficial for both psychological and physiological reasons. There is evidence that infants may benefit from sleeping in close proximity to their mother, and that not having the positive physiological regulatory effects from the mother may actually increase the risk of Sudden Infant Death Syndrome ⁵¹. For example, Basque social theories support the notion that bed sharing facilitates healthy personality development⁵². Broadly, by this theory, bed sharing is seen as appropriate for familial bonding ^28,53,54. In self-reports, some mothers explain that they bed share with their infants to aid breastfeeding ^50,55,56. The extended close contact of the mother and infant has also been associated with greater cardiac and respiratory function as well as better thermoregulation of the infant ^57,58.

Studies of sleep in parent-child bed sharing situations have focused on the mother and one child’s sleep, although the bed may be shared with another parent and/or child at the time. Although some mothers report that bed sharing improves their infant’s sleep and decreases crying ^56,59, there is evidence to the contrary. Some studies using actigraphy and sleep diaries have found that there is no difference in sleep between bed sharing and solitary sleeping infants ⁶⁰. Conversely, other studies using polysomnography support the claims that bed sharing leads to sleep fragmentation and lower quality sleep overall ⁴⁰. Bed sharing has also been associated with increased arousals throughout the night ^56,61,62 and shorter sleep durations for the child or infant in both subjective and objective sleep measures ^41,63.

While such effects of bed sharing are presumably negative in terms of infants’ sleep quality, it is possible that increased nightly arousals from slow wave sleep due to bed sharing are beneficial for prevention of Sudden Infant Death Syndrome. Since fewer awakenings are found in slow wave sleep, this stage is thought to pose a specific danger for infants by inhibiting them from awakening in response to airway obstruction or other dangers ⁶². Thus, although bed sharing may impair infant sleep, increased arousals may be a protective benefit of bed sharing for infants at risk of Sudden Infant Death Syndrome ³⁴.

Correspondingly, mothers perceive their own sleep to be improved by bed sharing with their infant. Subjective reports suggest sleep is improved based on feelings of comfort from the infant ^64,65, perceived decrease in awakenings due to crying during the night, and an increased feeling of restfulness by the mother from not having to ‘fully awaken’ to breastfeed when the infant is in bed with them ^56,64,66.

Counter to studies of subjectively measured sleep, studies using actigraphy and polysomnography find that maternal sleep is disturbed by the infant. In an actigraphic study in which maternal age, infant age, and infant weight were matched, bed sharing with infants resulted in more maternal awakenings throughout the night compared to sleeping alone. Co-sleeping mothers in this study experienced longer wakefulness during the night and had shorter continuous sleep episodes than solitary sleeping mothers ⁶⁰. Likewise, a study using polysomnography found that mothers have increased night awakenings when bed sharing with infants. Distinct differences in maternal sleep architecture are also found. Specifically, the amount of slow wave sleep is decreased, while the amount of stage 2 nREM sleep is increased among mothers who bed share with their infants ⁶². Taken together, these findings suggest that mothers who bed share have lower quality objectively measured sleep.

Overall, parent-child bed sharing has benefits and risks, particularly to infants. Bed sharing has generally positive effects on subjective sleep quality, and the choice to bed share may reflect psychological and cultural factors. Notably, in contrast to subjective measures, objective measures suggest both the mother and infant’s sleep is disturbed when bed sharing. Nonetheless, the long term effects of parent-child bed sharing – on sleep habits and on various aspects of development – are not yet understood, and despite the wealth of studies on mother-infant bed sharing, the evidence is limited ⁴⁰. An additional and important gap in the literature concerns father-child bed sharing. Research suggests that women’s sleep is more fragmented because of their role as a caregiver ^17,18, including breastfeeding, which provides a specific sleep disturbance for mothers that fathers would be lacking. Thus, bedsharing with infants may have less impact on fathers’ sleep.

Bed sharing between siblings

Sibling room sharing is common in families with multiple children. In two-thirds of United States homes, siblings share a room ⁶⁷. In Italy, 50% of children in one study were reported to share a room with a sibling⁴¹. Prevalence of bed sharing amongst siblings is less clear, and the research in this area is lacking (Table 3). However, studies using both subjective and objective measures to examine the benefits and drawbacks of crib sharing for twins ⁶⁸ and bed sharing in early childhood ²⁸ suggest this practice is not uncommon. For instance, studies conducted in England ⁶⁹ and New Zealand ⁷⁰ have indicated that approximately 54-60% of twins share their cribs within the first 4-6 weeks of infancy. Moreover, ongoing work in our lab suggests that, among a sample of over 500 preschool-aged (3-5 years old) children, 10.36% of children share a bed with a sibling. These children also make up 28.88% of the children who share a bed overall at this age, with the majority sharing a bed with parents ⁷¹. Some research suggests this may vary by race and ethnicity; African American children were more likely to share a room with a sibling than Latino and non-Latino White children. However, bed sharing was not reported ⁷².

Table 3.

Experiments examining sleep quality in bed sharing siblings

Authors	Study Population	Sleep Quality Measurement	Main Findings
Ball (2006)	60 families with infant twins	Completed sleep diaries for seven days when their infants were first and third postnatal months & semi structured phone interviews completed at the end of their infants’ first, third, and fifth months of life	No difference in sleep duration between bed sharing and solitary sleeping at the end of the first month. By the third month parents reported that solitary sleeping infants obtained significantly more sleep than their bed sharing counterparts. No significant differences in parent sleep duration were found.
Damato, Brubauker, & Burant (2012)	104 families with infant twins	Questionnaire and actigraph for parents at three time points: four weeks, eight weeks, and 12 weeks of age.	More than 65 percent of twins were bed sharing at four weeks, which decreased to 40 percent at 12 weeks. Mother’s sleep duration was lower when the twins were co-bedding vs not at four and eight weeks of age but not at 12 weeks. However, mother’s mean sleep quality was better when the twins were co-bedding than not, with the exception of 12 weeks of age.

In the extant literature on sibling bed sharing, bed sharing has been found to provide physiological and psychological benefits for newborn twins. Bed sharing helps twins to grow faster, to co-regulate, and to synchronize their sleep/wake cycles relative to solitary sleeping twins ⁷³. Twin bed sharing may improve infants’ sleep by reducing agitation; parents report that their twins are calmer when bed sharing and fall asleep faster compared to when they are separated.These psychological benefits are in accordance with findings discussed in other dyads. However, many of these benefits have been seen in neonatal units within hospitals in which the infants had oversight from nurses, decreasing the risks (e.g. Sudden Infant Death Syndrome ³⁴) associated with bed sharing.

Although bed sharing between older siblings and non-twin siblings is not prominent in the scientific literature, it is a popular topic for discussion among parents and caregivers. Anthropological data in the form of parenting blogs, forums, and popular culture articles provide a window into perspectives parents may have on sibling bed sharing. The benefits of sibling bed and room sharing include promoting social development and sibling bonding ^74,75. Additionally, some claim that kids have an easier time falling asleep when they share a bed ⁶⁷, particularly children who experience anxiety around bedtime ^67,75. Such a benefit may arise from the presence of the sibling alleviating fears of monsters and the dark ⁶⁸. Additionally, it is possible that having a sibling present when children wake up is enough of a reassurance that they do not feel the need to get out of bed and wake their parents. Another belief in support of sibling bed sharing is that having a comforting presence while falling asleep promotes good sleep habits and prevents sleep disorders from developing later in life ⁷⁶.

There are few statements made in opposition to sibling bed sharing. A common complaint found in the anthropological data is that keeping two kids in a room prevents them from getting an appropriate amount of sleep because the children will stay up late to play or talk ⁶⁷. Notably, this area lacks objectively-measured sleep studies to validate these concerns. Information regarding sleep fragmentation and sleep physiology in sibling dyads is not yet known but may be similar to the effects of parent-child bed sharing. Moreover, subjective reports in this literature are from the parent as opposed to the bed-sharers. In other words, future studies considering both objective sleep measurements and subjective sleep reports, particularly in older children who can accurately report, are needed and should be compared to sleep quality in children who do not sleep with siblings.

Bed sharing between owner and pet

Western dog and cat owners tend to develop strong personal relationships with their pets ⁷⁷. Thus, it is no surprise that pet owners often welcome their animals into spaces inside the home, including the bedroom. Nearly half of dog and cat owners share their bed with their pet ^78,79. Although growingly commonplace, few studies have explored the connections between pet bed sharing and sleep quality ⁸⁰ (Table 4).

Table 4.

Experiments examining sleep quality in owners bed sharing with pets

Authors	Study Population	Sleep Quality Measurement	Main Findings
Hoedlmoser, Kloesch, Waiter, & Schabus (2012)	330 children (mean age 9.52 years)	Self-report questionnaire (80 questions)	Found a significant association between sleeping with a pet and nighttime awakenings.
Hoffman, Browne, & Smith (2020)	16 female participants who shared the same bed with their dogs	Humans and dogs wore accelerometers for 14 consecutive days, CSD completed each morning	Co-sleeping dogs’ and humans’ movements over sleep were correlated, with more dog movements being positively associated with more human movements. Found that dog movements were more disruptive than disturbances unrelated to dog movements (meaning that the sleep disruptions related to the dogs lasted longer). Owners reported that they slept better with their pet than without.
Hoffman, Stutz, & Vasilopoulos (2018)	962 female participants	PSQI	Dog owners had earlier bed times and wake times than individuals with cats but no dogs. Dogs who slept in the owner’s bed were perceived to disturb sleep less and were associated with stronger feelings of comfort and security. Cats who slept in their owners’ bed were seen to be as disruptive as human partners but provided less security than human and dog bed partners.
Krahn, Tovar, & Miller (2015)	150 participants, 74 of which owned pets	Comprehensive sleep questionnaire and sleep interview	More respondents said that their pets did not affect/benefited their sleep. A smaller percentage of patients admitted that their pets had negative effects on their sleep.
Patel, Miller, Kosiorek, Parish, Lying, Krahn (2017)	40 healthy adults who shared the same bed with their pet	Humans and dogs wore accelerometers for seven nights	Human sleep efficiency was lower if the dog was on the bed as opposed to simply in the room
Smith, Browne, Mack, & Kontou (2018)	5 female participants	Humans and dogs wore accelerometers for seven nights, PSQI and ESS	Dog movement was a significant indicator of human movement, with humans being 4.3 times more likely to be awake when the dog was active. Co-sleeping caused sleep disturbances which led to detrimental sleep quality. Subjective measures did not reflect this detriment.
Smith, Thompson, Clarkson, & Dawson (2014)	1,018 adults who slept with pets who were matched with 1,018 that did not sleep with pets by age and sex	2012 Sealy Sleep Census (47 questions)	Participants who bed shared with pets took longer to fall asleep and were more likely to wake up tired than those without pets. No significant differences found in total-sleep length or feelings of tiredness during the day.

CSD, Consensus sleep diary (99); ESS, Epworth sleepiness scale (94); PSQI, Pittsburgh sleep quality index (89)

However, available data suggest that sharing a bed with a pet may have negative effects on sleep quality. Pet bed sharing has been associated with lower sleep efficiency and increased wakefulness after sleep onset as measured with actigraphy ⁸¹, as well as disrupted sleep associated with their pet’s movement ⁸². Conversely, a different study used self-report questionnaires and found that although the entire sample had poor sleep, the pet owners did not find their sleep quality to be significantly different than non-pet owners ⁸³. This difference may be because pets may be less likely to affect their owners’ sleep schedules than adult human bed partners, particularly those bed partners with different chronotypes and work schedules. Additionally, children (8-11 years) who share a bed with a pet were found to be more likely to report sleep problems such as awakening during the night and then staying up to play or watch TV afterwards, according to self-report⁸⁴.

Problems with bed sharing between humans and dogs, in particular, could relate to differences in consolidation of sleep bouts between domestic canines and our own species. While humans sleep monophasically, dogs sleep polyphasically and can have up to three sleep/wake bouts throughout the night, disrupting the sleep of their human co-sleepers when they wake ¹¹. Additionally, cats tend to experience their major sleep period in the mid-afternoon, so they may be more alert during the time when their human counterparts are asleep ⁸³.

Bed sharing with pets may be of particular concern to pet owners with allergies. Owners who are sensitive to pollen or dander, or who have asthma, may experience exacerbated allergic symptoms while sleeping in close proximity to an animal. Allergy symptoms such as cough, congestion, rhinorrhea, and watery eye are likely to make it difficult to fall asleep and to disrupt sleep throughout the night when bed sharing with pets ⁸⁵. However, some evidence suggests that early implementation of bed sharing with a pet may reduce allergies in children through incremental exposure to allergens ⁸⁶, possibly mitigating sleep disruptions due to pet allergies later in life.

Although some studies suggest that over half of pet owners report that their pets wake them up nightly ⁸⁰, the majority of owners do not believe that their pet disturbs their sleep ^81,87. Rather, they believe their pet aids in sleeping by providing comfort ^81,83,87. One study, which used actigraphy and self-reports, found that women had sleep disturbances which were correlated to their dog’s movements but still reported themselves as sleeping better with the company of their dog ⁸⁸. However, these discrepancies may be explained by differences in the age and size of the animal and the size of the shared bed. Generally, pet ownership affords psychological and physical benefits, collectively described as the ‘pet effect.’ Bed sharing increases time spent between owner and pet and may prolong these positive components of the pet effect. This may be one reason why owners choose to share their bed with their pet. Additionally, having a dog in the bed may serve as a protective factor and reduce the prevalence of some sleep disturbances, such as nightmares ⁸³.

Although bed sharing with a pet may provide a source of sleep disturbance, these deficits may be counteracted by the psychological and physiological benefits of bed sharing. Although direct comparison of pets and other dyads is lacking, it is worth considering whether pets may be the most amicable at adapting to a person’s sleep schedule, particularly in comparison to other dyads reviewed here. Comfort from pets may provide lasting effects on psychological health, an area only recently receiving attention. Examining whether these benefits outweigh the drawbacks will only increase in importance in the future.

Conclusion

Studies representing couple, parent-infant, and owner-pet dyads have conflicting results but generally find that bed sharing leads to increases in objectively-measured awakenings. Although increased awakenings are interpreted to indicate reduced sleep quality, subjective measures of sleep suggest that bed sharing in all dyads may improve perceived sleep quality through producing positive psychological benefits. These psychological benefits may be because, over time, humans have tended to sleep grouped together with others, including their friends, children, and pets. Communal sleep was actually considered safer because there would be more people to respond in the case of an emergency⁸⁹.

There is a disconnect between objective sleep quality (as measured with polysomnography or actigraphy) and perceived sleep quality (as measured through subjective measurements, such as self-report via questionnaires). Across the three of the dyads of interest (effects in siblings unknown), evidence tends to show that sleep quality is compromised when bed sharing, and yet people tend to report that their sleep is either better or unchanged. One study examining individuals sleeping alone found a correlation between subjective sleep measures, slow wave sleep, and awakenings. Specifically, more slow wave sleep and fewer awakenings predicted better subjective sleep quality reports in healthy individuals ⁹⁰. However, subjective sleep measures were not strongly correlated with sleep architecture. In this study, participants tended to underestimate their awakenings. A possible reason for this misjudgment could be that healthy sleepers may not be able to sense changes in their slow wave sleep and thus are unaware of their awakenings ⁹¹. An alternate explanation may be the difference in how polysomnographic waking periods are defined; polysomnography points to physiological indicators of wake that may not match the subjective experience. Additional future directions for research include conducting studies that utilize objective and subjective sleep measures for bed partners when sleeping alone and together. Although Drews and his colleagues have conducted a similar study ¹⁴, replication and increasing the sample size will be an important next step.

Although measures of sleep time and fragmentation suggest that bed sharing impairs sleep, there is insufficient research to understand the role of bed sharing on individual sleep stages due to the difficulty of using polysomnography on larger samples over a significant period of time. The little evidence available suggests that couples may have increased REM or slow wave sleep when bed sharing. However, new mothers may have reduced slow wave sleep. This lack of consistency in sleep physiology points to an important area for future research.

The positive sleep outcomes for subjectively measured sleep, in contrast to objectively measured sleep, may point to a biological need or desire for bedsharing that is often overlooked. The most obvious of these is the mother-infant dyad. As thoroughly reviewed by McKenna ⁵¹, mothers may have a biological drive to bed share with their infant for nurturing and feeding reasons that are fulfilled when bed sharing, even though fulfilling those needs may disturb objective sleep. This fulfillment may provide a positive lens on their sleep when they wake. Likewise, couple bedsharing may fulfill a biological drive (related to mating), and other pairs may fulfill a desire for safety, resulting in a more positive subjective view of sleep when bedsharing. This review points to the need for more research, including biological anthropology perspectives, on bed sharing.

Further, the gaps in the extant literature present several avenues for future inquiry. First, expansion of the demographics to include same sex couples and father-child dyads would be an important addition to the literature to improve representation of common dyads. Moreover, racial and ethnic differences in bed sharing practices have only been considered in parent-child bed sharing studies and should be a factor in future research. Longitudinal studies would be advantageous for investigating developmental effects of bed sharing on infants, as well as for understanding associations between sleep quality, relationship duration, and relationship quality in couples. Cross-cultural polysomnography studies would be advantageous for understanding any effects of cultural beliefs and norms on sleep architecture. Generally, more polysomnography studies that also incorporate subjective measures are needed across all dyad groups to fully establish the contrasting effects of bed sharing on sleep perceptions and architecture. The increased availability and quality of portable polysomnography devices (e.g., Sleep Profiler⁹², SOMNOtouch⁹³) may facilitate such research. These devices are less expensive and eliminate the need for costly technicians, thus allowing more data to be collected in-home simultaneously.

There should also be an increased focus on bed sharing with multiple bed partners, such as an individual sharing a bed with a human partner and a pet, which in one study occurred in more than forty percent of the sample ⁸³; two parents sleeping together with their child, which happens in most cases of bed sharing with babies ⁶⁶; or multiple siblings sharing a single bed. Finally, collection of data on the size of the shared bed and climate of the room may lend important insight into other variables that may affect sleep quality during bed sharing.

Almost all individuals will make the decision at some point in their lives on whether or not to bed share. Further research on bed sharing will inform decisions of when to bed share and when not to. The ultimate intent of this research should be to prevent sleep deprivation and optimize sleep benefit, which can have significant effects on other aspects of life outside of the bedroom, such as mood, health, and productivity. Balancing these needs with the psychological comfort that may come from bed sharing will be an important decision moving forward.