The study also found no differences in maternal mood between those mothers whose babies were sleeping for those longer blocks and those whose babies were not, though links between infant sleep practices and maternal stress have been found in other studies.

Douglas Teti, a professor of human development psychology and pediatrics at Pennsylvania State University, has studied the social criticism that mothers sometimes undergo if they are continuing to keep the baby in the parents’ room after the age of 6 months (the American Academy of Pediatrics recommends that for reasons of safety, infants should sleep in the parents’ room — but never in the parents’ bed — for at least the first 6 months of life, and ideally for the first year).

In our culture, he said, parents who continue to keep the baby in their room often face such criticism, while co-sleeping is the norm in many other cultures. Their studies have found that persistent co-sleeping is associated with less happy marriages and higher stress around co-parenting, though again, this is an association; there is no way to tease out cause and effect. Mothers, he said, “appear to be particularly vulnerable to losing sleep,” and may accumulate chronic sleep deficits, which may affect their well-being and their functioning as parents. “A lot is going to depend on how the parent is reacting or responding,” he said. “One of the things we tell parents is pay attention to your own sleep schedules, use good sleep hygiene.”

Some mothers can tolerate those deficits better than others, he said. “Not everyone is showing stressed co-parenting or stressed marriages.” Both members of the couple need to be on board with decisions about where the baby sleeps, and how night wakings will be handled, he said, and they should make sure they are taking time to nurture their own relationship.

The new study, like many other studies of variability and temperament and the different ways to take care of children, ought to be reassuring to everyone — children develop differently and there are lots of ways to grow up and be healthy. Parents who are stressed or distressed by an infant’s sleep pattern should talk to their pediatrician.

“When a mother asks me to teach her behavioral sleep techniques, I’m happy to do it,” Dr. Pennestri said, “but if a mother asks me to teach her that because she has pressure from a nurse or a friend telling her, ‘Your infant should sleep through the night,’ I don’t think she should use it.”

The authors of this study are clearly concerned that mothers are being pressured by unrealistic expectations of when babies will sleep through the night. In the world of infant sleep, that brings up highly polarized issues — and that polarization has unfortunately been less than helpful to parents everywhere.