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 Cosleeping Contribute to Lower Testosterone Levels in Fathers? Evidence from the Philippines ]