I sleep trained at 4.5 months. My baby stopped being able to fall asleep from nursing/patting/rocking/carrying.. so I was kind of left without a choice at that time. We let her cry it out. A sudden change in falling asleep technique of course made her cry. But we knew it works (my elder daughter was trained with the same method too) so I had to bear with the heart break for a while. But since then, aside from a few instances (regression, teething), my little one is put in bed, we close the curtains, turn on the fan and baby monitor, go out and close the door. She goes to sleep on her own. Sometimes she may wake up, but she self soothes pretty often, except when she’s really hungry or is in great discomfort. No negative psychological effects in both my children so far.