Moms aren't the only ones whose bodies change in response to their babies. A new study found that dads who share a bed with their children undergo a dip in their levels of testosterone, the so-called "male hormone." The decrease, researchers suggest, could influence how responsive bed-sharing dads are as parents.
"There's a growing body of literature that suggests that men's involvement in child care relates to lower testosterone," said study author and Notre Dame anthropologist Lee Gettler. (Gettler was affiliated with Northwestern University when the study was conducted.)
"We thought, if fatherhood is causing testosterone to decline, then maybe [co-sleeping] at night might also be related to their testosterone," Gettler said, explaining that the research is the first to probe the possible link between co-sleeping and men's hormone levels.
In the new study, published online in the journal PLoS ONE on Wednesday, Gettler and his colleagues analyzed data from more than 360 25- to 26-year-old dads living in a large city in the Philippines, where sharing the same room, if not the same bed, is common practice.
Indeed, more than 90 percent of the dads said they slept on the same surface as their children -- typically on mattresses or mats. The remaining fathers either slept in the same room as their children or were solitary sleepers.
When researchers took saliva samples before and after sleep, they found that the dads who shared a bed with their children had the lowest testosterone levels in the evening.
Gettler told The Huffington Post that he and his colleagues do not yet know why testosterone levels seem to take a dive after dads sleep in close proximity to their children, but he offered several hypotheses. There could be tactile cues or certain smells that affect testosterone, a hormone produced by both men and women (although men's levels are much higher). Though testosterone is usually thought of in relation to men's sex drive -- and indeed, it does affect libido -- the hormone has other functions. Some studies have linked higher testosterone levels to aggression, extroversion and risk-taking.
Conversely, lower testosterone has been linked to fathers' responsiveness to their children, the study's authors wrote.
"In an experimental study, men with greater testosterone reported lower sympathy or need to respond to infant cries relative to men with lower testosterone," they wrote. "And, in a separate, similar study, men's testosterone decreased in conjunction with providing a nurturing response to infant cries."
Last July, Gettler was an author on a study using the same group that found that when men became fathers, their testosterone levels dropped. And the more involved dads were in caring for their kids, the bigger the declines.
Gettler cautioned that his latest study is preliminary and does not establish clear cause and effect. For example, research has shown that changes in the amount and type of sleep men get can lower testosterone, which might account for some of the drops seen in the latest study. However, the researchers did attempt to control for disruption in their latest research.
In the U.S., bed sharing is a controversial practice. In its statement on infant sleep safety, the American Academy of Pediatrics, the U.S.' largest pediatric organization, recommends room sharing but not bed sharing, stating it does not "recommend any specific bed sharing situations as safe."
But a 2003 study found that the number of parents who regularly shared a bed with their babies is on the rise, up from just over 5 percent in the early 1990s to nearly 13 percent in 2000.
"Understanding how [co-sleeping] may impact physiology is important in understanding men's health and our thinking about fatherhood as a component of men's health," Gettler said. "There may be practical implications in terms of men's day-to-day well-being, as we know that testosterone is very important."