It seems that when fathers sleep near (or with) their infant and toddler children, they become biologically attuned to the children. The downside of this is that the testosterone levels of the father plummet - although this may be an evolutionary adaptation to keep the father engaged with the children and limit their desire to move on to the next female.

Full Citation:
Lee T. Gettler, James J. McKenna, Thomas W. McDade, Sonny S. Agustin, Christopher W. Kuzawa. (2012). Does Co-sleeping Contribute to Lower Testosterone Levels in Fathers? Evidence from the Philippines. PLoS ONE; 7 (9): e41559 DOI: 10.1371/journal.pone.0041559

Fathers Biologically Attuned to Their Children When Sleeping Nearby

ScienceDaily (Sep. 7, 2012) — Mothers aren't the only ones who are biologically adapted to respond to children. New research from the University of Notre Dame shows that dads who sleep near their children experience a drop in testosterone. Previous research from humans and other species suggests this decrease might make men more responsive to their children's needs and help them focus on the demands of parenthood.

In a recent study, Notre Dame Anthropologist Lee Gettler shows that close sleep proximity between fathers and their children (on the same sleeping surface) results in lower testosterone compared to fathers who sleep alone.

The study will appear in the September 5 issue of the journal PLoS ONE.

Gettler sampled 362 fathers, all of whom were between 25-26 years old, and divided them according to their reported nighttime sleeping location: solitary sleepers, those who slept in the same room as their children, and those fathers who slept on the same surface as their children.

Fathers' testosterone levels were measured from saliva samples collected upon waking and again just prior to sleep. Though the waking hormone levels of the three groups showed no significant differences, fathers who slept on the same surface as their children showed the lowest evening testosterone.

"Human fathers' physiology has the capacity to respond to children," Gettler says. "Our prior research has shown that when men become fathers, their testosterone decreases, sometimes dramatically, and that those who spend the most time in hands-on care -- playing with their children, feeding them or reading to them -- had lower testosterone. These new results complement the original research by taking it one step further, showing that nighttime closeness or proximity between fathers and their kids has effects on men's biology, and it appears to be independent of what they are doing during the day."

Substantial research has been conducted on the sleep and breastfeeding physiology of mother-baby co-sleeping, but this is the first study to examine how father-child sleep proximity may affect men's physiology, and it is the first to explore the implications of co-sleeping for either mothers' or fathers' hormones.

In other species, testosterone is known to enhance male mating effort through its influence on muscle mass and behaviors related to competing with other males and attracting female attention. The hormone is thought to operate similarly in humans, and higher testosterone has been linked to behaviors that might conflict with effective fathering, such as risk taking and sensation seeking. Prior research found that men with lower testosterone reported greater sympathy or need to respond to infant cries relative to men with higher testosterone.

"There are so many intriguing possibilities here for future research: Why do fathers have lower testosterone when they sleep very close to their children? Does it reflect human fathers' roles in our evolutionary past? How much do fathers vary in their nighttime care when their kids are close by? How does co-sleeping change fathers' sleep architecture when we know that co-sleeping increases mothers' arousals and mothers sync to their infants' sleep patterns," says Gettler.

"Testosterone is a hormone that frequently is a part of public discourse, but the false idea that 'manliness' is exclusively driven by testosterone often dominates the conversation. There is growing evidence that men's physiology can respond to involved parenthood -- something that was long thought to be limited to women. This suggests to us that active fatherhood has a deep history in the human species and our ancestors. For some people, the social idea that taking care of your kids is a key component of masculinity and manliness may not be new, but we see increasing biological evidence suggesting that males have long embraced this role."