What does that mean, when a newborn baby smiles during his or her sleep?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions. No reason is given in Shariah about the smile of a newborn baby. May be he smiles when he is in good health, satisfied with his suckling, feels comfortable in his bed and good clothes, etc. However, pediatrists and other children specialist may give more comprehensive answers to this question in this regard. Allah knows best.