Why do babies smile when their asleep?

I was told that it could be "dreams" or the physiological changes that the mommy above mentioned which triggers certain muscles or it could also be gas. My niece makes a smiling face when she poops in her sleep. This will quickly turn into a crazy loud cry soon after.

I have read that a baby would go through some physiological changes during his/her REM sleep. These physiological changes would activate certain reflexes and smiling is one of those reflexes that gets activated.

Mine smiles and laughs in her sleep! And sometimes cry damn loud from a bad dream; that always make me wonder what do babies even dream of lol.