New dads might expect to deal with dirty diapers or a baby's cries in the night. But how about lower testosterone levels? Previous research has shown that fatherhood can indeed cause a man's testosterone to dip. And that may be especially true for dads who sleep next to their babies, according to a new study in the journal PLoS ONE. [Lee T. Gettler et al., Does Co-Sleeping Contribute to Lower Testosterone Levels in Fathers? Evidence from the Philippines]

Researchers studied 362 Filipino fathers of the same age. They took testosterone samples in 2005 and 2009. And they found that dads who slept next to junior had a third less testosterone than before, compared to dads who dozed alone.

The researchers can't be sure that sleeping next to the baby actually causes that decline. Could be that dads with less testosterone are just more likely to cuddle with the kid. But they say disruptions in a man's sleep—a baby's cry, for example—are known to cut testosterone production.

Either way, the authors say, less testosterone is probably better for baby. Because high levels of the hormone are linked with aggression, risk-taking, and less sympathy for an infant's cries—which, at three in the morning, probably won't make mom a happy camper either.

—Christopher Intagliata

[The above text is a transcript of this podcast.]