Why do fathers' testosterone levels drop when sleeping near their children?

Last year, we reported on the strongest evidence yet that becoming a father is linked to a drop in a man's testosterone levels. Now it seems a family's sleeping arrangements actually have an even more specific effect on dad's biology.

Notre Dame anthropologist Lee Gettler is one of the authors of last year's paper, which found that males with relatively higher testosterone levels were more likely to father children, but they then experienced a drop in testosterone level after having kids that was far greater than the decline in their childless counterparts, suggesting an actual causal link between fatherhood and testosterone levels.

Now Gettler has published a new paper that drills down on this link still further, and this latest study suggests a father that sleeps on the same surface as his child, have lower nighttime testosterone than those who just sleep in the same room as their child, and both have lower testosterone than those who sleep in separate rooms. Gettler tested the testosterone levels of 362 fathers, all between the ages of 25 and 26 at the outset of the 4.5-year study. Those who slept on the same surface as their children experienced a significantly greater decline in testosterone over the course of the study than their peers. The waking testosterone levels didn't vary significantly between the groups.

One point that should be brought up about this study is that the subjects were drawn from the city of Cebu in the Phillippines, in which a whopping 92% of the participants said they slept on the same surface as their children. That's a very different distribution of sleeping arrangements than what we're used to, and a question worth considering is whether there's enough representation in the other two categories in this sample to really assess meaningful differences. Gettler acknowledges the potential for sample size issues in the original paper, which is freely available over at PLoS ONE, and it's well worth checking out to see the complete thinking behind the study.

Still, even if the study's conclusions can't be considered definitive proof of this apparent effect — and Gettler and his team never make any such claims in their paper — it's still intriguing to wonder just what this all might mean. In a statement, Gettler gets the ball rolling with some fascinating ideas about what could be behind this effect:

"Human fathers' physiology has the capacity to respond to children. 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.

"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."

Via PLoS ONE. Image via Enigma Photos's photostream