Sleep training continues to be a popular parenting philosophy with its roots in American Behaviorism. The idea behind this approach is that if you want to stop a behavior, don’t “reinforce” it, which increasing its likelihood that it reoccur. Under this school of thought, if you pick up a crying baby, you are just reinforcing the likelihood that the baby will cry more often—especially at night. Recent research has suggested that there is “no apparent harm” to this approach. But is that true? This presentation will describe recent research in neuroscience on the importance of responsive early parenting on brain development, and harmful effects of leaving babies to cry. Non-responsive parenting raises babies’ cortisol levels. Cortisol is toxic to brain cells, particularly in the hippocampus. This research is also put in the context of current AAP recommendations to keep babies in the parents’ room during the first six months, and how solitary sleep increases the babies’ risk of SIDS.