Have a read through of wellrested's excellently communicated point on this thread www.mumsnet.com/talk/sleep/4722523-what-does-crying-it-out-actually-mean-practically?page=2
This is a great article too, also linked to in that thread www.nytimes.com/2019/04/19/opinion/sunday/baby-breastfeeding-sleep-training.html
"This idea comes, perhaps surprisingly, from 1980s Romania, where thousands of children lived in orphanages with very little human contact for months or even years. One of the things visitors noticed in these places was the eerie quiet. Babies didn’t cry, because they knew no one would come. The argument is that “cry it out” does the same thing.
This is absurd. Sleep training methods do not leave the infant for months without any human contact, nor do they suggest subjecting children to the other types of physical and emotional abuse that occurred in those orphanages.
To learn about the impact of sleep training, we need to study it in the way it is actually used. Fortunately, many people have, and in a lot of those cases they used randomized trials.
Consider an Australian studydy of 328 mothers whose 7-month-old babies were having problems sleeping. Approximately half were assigned to do a sleep-training regimen, and the others were not. In the short term, the authors found significant benefits: The intervention improved sleep for children and also lowered parental depression. But they didn’t stop there.*
They returned to evaluate the children a year later and five years later, when the children were 6. In this later follow-upup, which included a subset of the original families, the researchers found no difference in any outcomes, including emotional stability and conduct behavior, stress, parent-child closeness, conflict or parent-child attachment. Basically, the kids who were sleep-trained looked exactly like those who were not.
These results are not an outlier. Review studies of sleep-training interventions do not find negative effects on infants. And many show sizable improvements in maternal depression and family functioning. Sleep affects mood, and parents who sleep less feel worse. The evidence paints a pretty pro-“cry it out” picture.
Nonetheless there are academic articles that argue against it. One small study that gets a lot of playay_ shows that in the few days after sleep training, mothers are less stressed, but the same is not true of infants. The researchers interpret this as a signal that the mothers and children are losing emotional touch with each other, but this is a stretch. Why not interpret the evidence to say that cry-it-out relaxes parents without hurting children?
Fundamentally, the argument against sleep training is theoretical: that some children are devastated, even if those results don’t show up in the data, or that the damage may not manifest until babies are adults.
I think it is fair to say that it would be good to have more data. It’s always good to have more data! However, the idea that this uncertainty should lead us to avoid sleep training is flawed. Among other things, you could easily argue the opposite: Maybe sleep training is very good for some kids they really need the uninterrupted sleep and there is a risk of damaging your child by not sleep training."