New research suggests that fathers are biologically adapted to their children when they sleep near them.

Researchers at the University of Notre Dame found that dads experience a drop of testosterone when they sleep in close proximity to their children.

Previous studies done on from humans and other species suggest that this decrease might make men more responsive to their children's needs and help them focus on the demands of parenthood

"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."

For the study, researchers collected data from 362 men, all of whom were between 25 to 26 years old. They were divided in to three groups 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.

They used saliva samples to measure the fathers' testosterone levels. 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.

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.

The findings are detailed in PLoS One.