science

Plenty. If you scroll down, there's tens of research articles linked. You just have to click on the circles for most of the articles :-)

Here's an excerpt from the bottom of the article':

The most conclusive long-term study on sleep training to date is a 2012 randomized controlled trial on 326 infants, which found no difference on any measure—negative or positive—between children who were sleep trained and those who weren’t after a 5 year follow up. The study includes measurements of sleep patterns, behavior, cortisol levels, and, importantly, attachment.