The post by @Snowoctopus I’m sure is meant well, but isn’t really very fair on parents who sleep train their children. Totally respect that it’s not for everyone, but to say that children who are sleep trained wake in the night but don’t cry, instead they lie there scared alone simply isn’t true, and I just want to add an alternative perspective. Children still wake up, yes, and if they need anything or are in pain they will still cry. Ask any parents who have done sleep training - there are always still terrible nights with illness and teeth.
There have been studies done where children who are neglected and never receive responsive, nurturing care do learn not to cry for anyone, because no one is coming. There is one study in particular that is often referenced but this involved children in Romanian orphanages. I have worked with children like that, so this is something I have seen first hand. But that simply isn’t comparable to sleep training a child who is loved and cared for. There are also plenty of studies showing no negative effects from sleep training at an appropriate age.
Children develop secure attachments with their parents through every single interaction with them. Every responsive interaction you have with your child where you connect with them, show them love, interpret, understand, and meet their needs - that builds security, a sense of safety, and a happy, healthy parent child relationship. Think how many hundreds of times a day you do that! Parents’ responses to their children need to be ‘attuned’ over 50% of the time, to lead to the child developing a secure attachment style. The point I’m trying to make is that a child’s sense of security, their feeling that they can trust and rely on their parent, cannot be broken by a couple of nights of sleep training. Children are more resilient than that. The child-parent relationship is more robust than that.
To be clear, I’m not trying to persuade anyone to sleep train their child, just want to reassure you that if done appropriately and in the context of a nurturing, loving parent child relationship, it won’t do any harm.
The book ‘crib sheet’ also gives an overview of the research done on controlled crying as a form of sleep training. Really hope this is helpful.