Have noticed the 1.5 month old smiling when asleep.
Do newborns have dreams when they sleep? If yes, then what kind? If no, then what could be the reason of their sleep smiles?
At that age smiling is often associated with gas.
As I mentioned in another answer, the question of whether or not newborns dream has not been conclusively answered. From that answer:
Do Babies Dream: Generally people go through five stages when they sleep. Four stages belong to non-REM (Rapid Eye Movement) sleep and one stage is REM sleep. REM sleep is the stage of the sleep in which we dream. Babies will spend most of their sleep time in this phase i.e.REM sleep.
According to the recent studies, it is proved that babies don't dream but they learn while sleeping. They use REM sleep phase to develop new brain pathways and later develop language i.e. they process knowledge while they sleep.
Baby's Smile When They Sleep: Actually baby needs approximately 8 weeks for their brains to develop in such a way to respond emotionally to external stimuli. Below that baby's smile is nothing but "gassy smile".
If the cheeks go up and the eyes squint then it is a "social smile" whereas "gas smiles" will be smaller and they will not lift the cheeks up.
REM sleep phase is an active period for passing gas as well. So babies smile while sleeping i.e. gassy smile.