Hands down, the biggest mom fight I ever witnessed involved two Upper West Side parents debating the pros and cons of the "cry-it-out" sleep method right before a Mommy and Me class. The young instructor, who appeared ready to cry it out herself, had to break up the fight so she could start class. Cry-it-out is a sleep training method that advocates letting your baby cry (or the more innocuous-sounding "self-soothe") for varying periods of time before offering comfort. The goal is to get your baby to learn how to fall asleep on her own, so you, too, can rest. Central to it all is stress and sanity: the baby's, yours and that of everyone with earshot. The method is the subject of intense debate, passionate opinions and conflicting research findings. A few weeks ago, the journal Developmental Psychology published a study supporting the notion that a majority of infants over the age of 6 months may best be left to self-soothe and fall back to sleep on their own. Noting that sleep deprivation can exacerbate maternal depression, Temple University researcher and professor Marsha Weinraub concluded: "Because the mothers in our study described infants with many awakenings per week as creating problems for themselves and other family members, parents might be encouraged to establish more nuanced and carefully targeted routines to help babies with self-soothing and to seek occasional respite." There is broad agreement that parents' well-being is critical to infants' health and development. Weintraub suggested that the link between infant awakenings and maternal depression would benefit from further research. Adequate sleep is, of course, key to parents' stress levels. Loss of sleep has been associated with a dramatically higher risk of depression in mothers and marital problems. It is how well (or not) the baby fares in the cry-it-out scenario that muddies the waters. On the pro sleep-training side, an Australian study published in September followed 326 children with parent-reported sleep problems at 7 months. Half the babies were placed in a sleep-training group and the other half in a control group that did not use sleep training. Five years later, researchers followed up with the now-6-year-old participants and their parents. The children in the two groups showed very little to no significant differences in terms of emotional health, behavior or sleep problems. Mothers' stress or depression levels were roughly the same, as were the parent-child bonds in both groups. The researchers found no harm in permitting children to cry for limited periods of time while they learned to sleep on their own. Directly contradicting this study is research conducted at the University of North Texas that was published in the Early Human Development journal last year. Observing 25 infants aged 4 to 10 months in a five-day inpatient sleep training program, researchers monitored levels of the stress hormone cortisol in the babies, who were left to cry themselves to sleep without being soothed. The scientists measured how long the infants cried each night before they fell asleep. The mothers sat in the next room and listened to their children cry but were not permitted to go in and soothe their babies. By the third night, the babies were crying for a shorter period of time and falling asleep faster. However, the cortisol levels measured in their saliva remained high, indicating that the infants were just as "stressed" as if they had remained crying. So while the infants' internal physiological distress levels had not changed, their outward displays of that stress were extinguished by sleep training. In the mothers, on the other hand, the stress hormone levels fell as the babies appeared -- at least outwardly -- to settle down and sleep. The study did not clarify whether the babies' stress levels lowered as their sleep patterns settle over time. The researchers are now studying this issue, among others, in a longer follow-up. As with most things in life, when it comes to babies and the science of sleep, the only certainty is that there is no certainty. Those of us on the roller coaster of modern parenting are the first to attest to the fact that perfection simply does not exist, especially when you're bleary-eyed and sleep-deprived at 4:15 a.m., with a full workday looming. Some researchers suggest that parents may gain clarity by working backward from a longer-term goal. Darcia Narvaez, professor of psychology at the University of Notre Dame, studies moral cognition and development. Her research examines how early life experience may influence brain development, moral functioning and character in children and adults. Narvaez advocates a more responsive style of parenting that mirrors nurturing ancestral practices, including breastfeeding, frequent touch, soothing babies in distress, outdoor play and a wider community of caregivers. According to Narvaez, research shows that responsive parenting can help develop infants' self-regulation and may influence conscience, impulse control, empathy, resilience and other character-related attributes. Narvaez's list is strikingly similar to a set of character traits discussed by journalist Paul Tough in his book, "How Children Succeed: Grit, Curiosity, and the Hidden Power of Character." In the book, Tough examines the skills and traits that lead to success and ultimately advances the hypothesis that character attributes may be more crucial than cognitive skills like IQ and intelligence. "(I)n the past decade, and especially in the past few years," writes Tough, "a disparate congregation of economists, educators, psychologists, and neuroscientists have begun to produce evidence that ... (w)hat matters most in a child's development ... is not how much information we can stuff into her brain in the first few years. "What matters, instead, is whether we are able to help her develop a very different set of qualities, a list that includes persistence, self-control, curiosity, conscientiousness, grit and self-confidence." Can responsive parenting in a child's first year lay the groundwork for better regulation of social and behavioral responses and perhaps even greater life success? Seems like a heavy burden. And no one knows for sure -- not even the dueling Upper West Side mothers.