Before my daughter was born, I was a music therapist in a children's hospital. My job was to go room to room to help kids who were in pain and/or scared, among other things. They often needed to sleep more than anything, so many days, I went from room to room singing babies and kids to sleep.
This is where the "ISO Principle" comes in. "ISO" means "equal, similar, identical" in Greek. The ISO Principle is a music therapy technique where you match the music you're using to the mood and arousal level of whoever you're trying to help.
We had some visitors stay at our house and they brought their sweet 18-month old. He was sick, missed a nap and was overstimulated after a long day in the sun. Predictably, he had a rough time calming down for a nap. Before long, it escalated to him being completely inconsolable. The method I used is something I've used time and time again... and within minutes, he was fast asleep.
When I think back to my daughter's first few years, I am almost immediately taken back to the quiet nights when I rocked her to sleep while singing her special lullaby.