My girl used to do this too, and the first time I heard it I was so startled I rushed over to check on her. From what I have read and what our PD mentioned, babies go through more active sleep phases and laughing or smiling in their sleep is generally considered normal and not something to worry about. That said, if it is happening alongside other things like stiffening or jerking movements, it is worth bringing up at your next polyclinic visit just to be sure. Most of the time though it is one of those sweet little moments that actually means your baby is doing fine.