Should You Let Your Baby Cry? What Science Says About Sleep Training

When a baby is left to cry for a period without anyone attending to them, a hormone called cortisol may rise in their body, which is responsible for the feeling of stress. Even if the baby stops crying, this does not mean their stress has ended, because their body may remain in a state of internal tension.

However, doctors say that using the "let them cry" method occasionally and in a controlled way, such as letting the baby cry for a few minutes and then soothing them, does not harm their mental or emotional development. The important thing is that this is done in a safe and loving environment.

The real danger is constant neglect and not responding to the child's feelings for a long time, as this can negatively affect their development.

In short:

Crying for a short period is not dangerous if the parents generally care for the child.

The danger is continuous neglect and lack of emotional interaction with the child.

source
La Leche League Great Britain. (n.d.). Letting babies cry – The facts behind the studies. Retrieved from https://www.laleche.org.uk/letting-babies-cry-the-facts-behind-the-studies/