Med thumb dog and cat in bed full size

Co-sleeping is an extensively studied practice and a hot-button topic in the sleep world. But the term isn't always clearly defined. Co-sleeping can refer to bed-sharing between two adult partners or, as is most often the case, between parents and young children. In fact, co-sleeping may even be broader than bed-sharing: Some researchers have stipulated that parent-child co-sleeping occurs whenever a parent and child are "in close enough proximity to exchange at least two sensory stimuli, such as touch, smell, movement, sight and/or sound."

Different definitions of co-sleeping, however, are consistent in at least one way: They imply that co-sleeping is a humans-only activity. And that's a problem, according to psychologists at Central Queensland University, who made their case in a new review paper published in the journal Human Nature.

Humans have shared their sleeping quarters with animals for a few centuries, at least. And, today, roughly half of people who own pets let them in their beds. So it's time, the Central Queensland team says, to recognize that falling asleep with a puppy in your arms counts as co-sleeping — and to buckle down on studying the topic. "Given that sleep accounts for a large portion of human and animal life, and that interspecies co-sleeping impacts humans, animals, interpersonal relations, and interspecies relations," researchers wrote, "there is an urgent need for researchers to truly contemplate “who’s been sleeping in your bed?”'

To emphasize how naturally human-and-animal co-sleeping fits into the larger co-sleeping conversation, researchers used the same overarching factors that drive parent-child co-sleeping decisions to evaluate the debate over curling up with pets. It's not a perfect fit — SIDS and fur-covered-pillows are hardly comparable risks. But, there are similarities between the motivations for, and potential concerns about, co-sleeping with children and animals.

Social sleep and three-dog nights

In Western, industrialized nations, the prevailing view on parent-child co-sleeping is: Don't do it. But, in many non-Western societies, co-sleeping is the norm. And it was the norm in Europe leading up to the Industrial Revolution. Before Victorian austerity, urban over-crowding, artificial light and disposable wealth entered the picture, children learned to sleep alongside their parents, siblings, extended family members and even house guests. The rise of Industrialization subsequently coincided with the fall of "social sleep."And, with time, co-sleeping went from the presumed nocturnal arrangement to something derided as outdated, unhealthy and kind of weird. 

Unlike our extensive knowledge of parent-child co-sleeping, the historical record of human-animal co-sleeping is meager. There's some evidence that pet co-sleeping has been a thing for a few centuries. The phrase "Three-dog night," a common Australian idiom for a night so cold you need to sleep near three dogs to stay warm, is thought to have origins in either the Chukchi people, who lived in Siberia between the 17th and 19th centuries, or the Australian outback. The latter explanation jibes with limited accounts of Aboriginal Australians using dogs and/or dingoes to stay warm at night and ward off evil spirits. And, in preindustrial England, dogs and cats respectively hung out in and around sleeping quarters at night to keep people secure and keep rodents out.

The same psychological needs motivate people to give up pillow space for both furry heads and tiny bald ones.

Today, human-animal co-sleeping appears to be most prevalent in Western cultures. One 2011 study on 60 non-Western societies showed that pets (primarily dogs and cats) were equally as likely to sleep outside, inside away from people and inside alongside or near people. Most societies reported keeping dogs as pets. But only seven of them allowed dogs indoors; and only six let dogs sleep inside at night. By contrast, remember that half of pet owners have four-legged bed partners, based on the most recent estimates from media and consumer surveys. 

So, parent-child co-sleeping fell out of favor in the same societies where pet co-sleeping has become a widespread practice. We might know more about person-pet dozing duos if the topic received a fraction of the academic attention given to parent-child co-sleeping. But the limited human-animal literature that does exist, study authors argue, suggests that the two varieties of co-sleeping are analogous in several ways: Allowing babies and dogs into bed comes with overlapping (though not identical) considerations related to health, sleep quality, behavior and sex. And, in the end, the same psychological needs motivate people to give up pillow space for both furry heads and tiny bald ones.  

Health

Babies
Parent-child co-sleeping has become anathema in the US mainly because it's linked to infant injury and death caused by SIDS and accidental suffocation. Some experts argue that other factors, including the use of improper bedding and parental drinking and smoking, have thrown off the co-sleeping data.

Pets
Health risks associated with pet co-sleeping include transmission of zoonotic diseases, allergies, asthma and the general notion that having a dog in bed is unhygienic. These are principally human-centric concerns. But, given that two-thirds of human diseases are zoonotic and can infect animals, it's possible that bed-sharing dogs could catch their owners' bugs, too. Although, study authors explain, fear over disease transmission may be overblown. The risk of humans contracting zoonotic diseases from sleeping with animals is low, particularly when animals are clean and receive regular veterinary care. Fretting over brindle fur on pima cotton, on the other hand? Totally valid. 

Sleep impairment

Babies
Studies suggest that adults get worse, more disturbed sleep when they share their beds (with anyone). Daytime functioning may be affected as a result, depending on the extent and frequency of the disturbances. And babies appear to wake up more frequently during the night when they sleep with their parents. It's not clear that it's wholly negative, however, for parents to be privy to babies' nighttime awakenings, as they could signal health issues that would otherwise go undetected. 

Pets
Adults might experience comparable sleep disturbances from sharing beds with dogs and babies. For one thing, humans' sleep cycles are mismatched with those of their canine companions. Dogs, who are polyphasic sleepers, fall asleep and wake up more frequently than their monophasic owners. And, compared to humans, dogs remain more responsive to environmental sounds when they're asleep, making them potentially active bed-partners.

In one 2014 survey of Australian pet-owners, co-sleeping was associated with difficulty falling asleep and increased awakenings caused by animal noises. An American study from the same year had similar results: Nearly one-third of co-sleeping pet-owners said they were woken up by their pets at least once per night. Unfortunately, we don't have a great sense of how or how much sleep disturbances from human-animal co-sleeping actually impairs pet-owners' daytime functioning. The limited existing research doesn't go into that level of detail. 

Behavior

Babies
There are concerns that co-sleeping both leads to and reinforces poor behavior in children. Some studies do essentially say that co-sleeping kids become anxious, overly dependent poor sleepers. But, in other studies, co-sleeping has been associated with positive qualities like self-reliance. 

Pets 
Co-sleeping could also contribute to problematic behavior in pets. Dogs who sleep in their owners' beds, according to some research, may have more accidents and display more aggression towards other pets in the household. Co-sleeping may also cause or exacerbate separation anxiety in dogs. But, again, we don't have enough research on co-sleeping dogs to know if observed behavioral problems start before or after bed-sharing does.

Sex

Babies
Co-sleeping a) restricts parental opportunities for sex and intimacy, potentially affecting relationship quality, and b) exposes children to adult sleepovers, leaving them vulnerable to incest and psychological scarring. There is some literature to validate both of these concerns. In one 2008 study, parents who co-slept with older children reported more marital distress than parents who kept their doors locked at night. And psychologists have suggested that, by exposing kids to sexual activity without any understanding of the context, co-sleeping leaves could encourage children to imitate adult behavior. 

But there's also research to the opposite effect. In a 2007 study, there was no difference in relationship satisfaction between parents who made their beds adult-only territory and co-sleeping ones. And a study that tracked children from infancy through adolescence showed no connection between infant co-sleeping and subsequent sleep problems, sexual dysfunction or other residual psychological trauma. 

Pets
As for pets? Researchers have suggested that bed-sharing animals interrupt couples' sex lives, particularly when only one partner wants the pet in the bed. But there's not much in the way of evidence to support this belief. And, while co-sleeping with animals could potentially open the door for beastiality, researchers feel there's "little reason to link zoophilia among pet owners as a prevalent and normative motivation for co-sleeping." 

Motivations

Babies
Across the board, it's common for people who choose to co-sleep, with their children or partners, to say the practice provides comfort, emotional support and companionship. And, when it comes to children, researchers have identified two types of co-sleeping parent: early and reactive. Early co-sleepers embrace the practice from the get-go, often for philosophical or cultural reasons. But practical concerns (like lack of space) come up, too. 

Early co-sleepers are likely to report satisfaction with their sleep arrangement and see the practice as a form of bonding. Based on one 2002 study, researchers proposed that co-sleeping can help fathers, in particular, overcome feelings of distance with children. And working mothers have said that co-sleeping "can account for lost time with their infants during the day, validate their maternal role, and ensure that their infants know that their mothers love them and want to be with them."

Pets 
We only have a limited understanding of what motivates human-animal co-sleeping. But, "as in parent-infant co-sleeping," researchers wrote, "the decisions made by a pet owner about where their pet sleeps during the night are dependent upon philosophical, psychological, and cultural orientations, as well as emotional and practical factors."

Pet owners can be divided into the same early and reactive categories: Early co-sleepers want to be woken up by puppy kisses; reactive ones let their pets into bed to alleviate bad behavior, such as whining. For pet-owners who enjoy it, "co-sleeping may provide or enhance psychological benefits," explained study authors. "Yet the opposite may hold true for some owners. For example, those who are lonely may be more likely to sleep with their pets, and whilst this may be comforting, an unhealthy pathological level of pathological attachment may ensue." 

It's reasonable to assume that co-sleeping pet owners could be motivated by an unconscious desire to feel closer to their pets. One 2016 study on Dutch-speaking pet owners offered preliminary evidence that dog-owners who let pets sleep in their bedrooms show higher levels of global attachment than non-co-sleepers. Pet-owners who only see their pets after work, researchers posited, may feel a similar need as working parents to maximize quality time through shared sleep. What do we know about the emotional lives of pet-owners who let their animals, but not their human children, share their beds? Very little. It's one of many unexplored issues.

Is co-sleeping the same practice with children and pets? Of course not. Unlike kids, dogs don't age out of sharing their (pet) parents' beds. And, it's probably worth repeating, experts primarily advise against co-sleeping with infants to keep them alive and safe. With dogs, opposition is more about tracking dirt and hogging bed real estate. Regardless, it's worth learning more about a practice that millions of people do every night, and which likely affects humans' lives and well-being. Plus, sleeping dogs make for adorable study subjects.