Sleep like a Baby!

Why do we someone is "sleeping like a baby?" Especially when babies generally don't sleep through the night??

I guess it has to do with the peaceful way, otherwise active children appear when they are sleeping. They relax completely, and look like they don't have a care in the world. They look like little angels. Maybe you have to be a parent to know what I'm talking about. I love watching my child sleep, and I think most parents do.