Proponents hold that co-sleeping saves babies' lives (especially in conjunction with nursing), promotes bonding, lets the parents get more sleep, facilitates breastfeeding, and protects against sudden infant death syndrome (SIDS). Older babies can breastfeed during the night without waking their mother.
Opponents argue that co-sleeping is both stressful and dangerous for a baby, pointing to evidence that co-sleeping may increase the risk of SIDS, They also cite concerns that a parent may smother the child or promote an unhealthy dependence of the child on the parent(s). In addition, they contend that this practice may interfere with the parents' own relationship, by reducing both communication and sexual intercourse at bedtime, and argue that modern-day bedding is not safe for co-sleeping.
One study reported mothers getting more sleep by co-sleeping and breastfeeding than by other arrangements.
It has been argued that co-sleeping evolved over five million years, that it alters the infant's sleep experience and the number of maternal inspections of the infant, and that it provides a beginning point for considering possibly unconventional ways of helping reduce the risk of SIDS.
Stress hormones are lower in mothers and babies who co-sleep, specifically the balance of the stress hormone cortisol, the control of which is essential for a baby's healthy growth.
In studies with animals, infants who stayed close to their mothers had higher levels of growth hormones and enzymes necessary for brain and heart growth.
The physiology of co-sleeping babies is more stable, including more stable temperatures, more regular heart rhythms, and fewer long pauses in breathing than babies who sleep alone.
Co-sleeping may promote long-term emotional health. In long-term follow-up studies of infants who slept with their parents and those who slept alone, the children who co-slept were happier, less anxious, had higher self-esteem, were less likely to be afraid of sleep, had fewer behavioral problems, tended to be more comfortable with intimacy, and were generally more independent as adults. However, a recent study (see below under dangers) found different results if co-sleeping was initiated only after nighttime awakenings.
Co-sleeping is known to be dangerous for any child when a parent smokes, but there are other risk factors as well. Some common advice given is to keep a baby on its back, not its stomach, that a child should never sleep with a parent who smokes, is taking drugs (including alcohol) that impede alertness, or is obese. It is also recommended that the bed should be firm, and should not be a waterbed or couch; and that heavy quilts, comforters, and pillows should not be used. Young children should never sleep next to babies under nine months of age. It is often recommended that a baby should never be left unattended in an adult bed even if the bed surface itself is no more dangerous than a crib surface. There is also the risk of the baby falling to a hard floor. Parents who roll over during their sleep could inadvertently crush and/or suffocate their child, especially if they are heavy sleepers and/or obese.
A recent report suggests that co-sleeping initiated after night awakenings and other parenting behaviors, such as holding a baby until it falls asleep or remaining in the room until a baby sleeps, interferes with a baby's abilities to learn to comfort itself. Babies who had been exposed to co-sleeping or related parental behaviors had significant problems with sleep later in life. These difficulties with sleep were associated with increased health risks in older children .
There are several products that can be used to facilitate safe co-sleeping with an infant.
A study of a small population in Northeast England showed a variety of nighttime parenting strategies and that 65% of the sample had bedshared, 95% of them having done so with both parents. The study reported that some of the parents found bedsharing effective, yet were covert in their practices, fearing disapproval of health professionals and relatives. A National Center for Health Statistics survey from 1991 to 1999 found that 25% of American families always, or almost always, slept with their baby in bed, 42% slept with their baby "sometimes", and 32% never co-slept with their baby.