play sleep
The term 'play sleep' does not have a direct translation in German. However, 'Schlafspiel' is a compound word that could be used to describe a game or activity related to sleep. This term is not commonly used and its interpretation may vary.
Their daughter's sleeping game is very interesting.
This sentence is indicating that the daughter of the person they're talking about has a sleeping game (Schlafspiel) that is considered to be interesting. Without context, we cannot definitively say what exactly this sleeping game refers to, but it could be a particular routine, ritual or pattern the daughter follows when she is sleeping or preparing to sleep.
He found her sleep game frightening.
The person they're talking about finds a woman's (or girl's) sleeping game (Schlafspiel) to be frightening. Again, without context, we cannot definitively explain what the sleep game refers to but it may mean a particular behavior, pattern or event that happens when this woman is sleeping or preparing to sleep, that the person finds scary.
I can't understand your sleep game.
In this sentence, the speaker is directly addressing someone else and stating that they are unable to understand the other person's sleeping game (Schlafspiel). As before, this could mean any particular routine, habit or pattern associated with the other person's sleep that the speaker is unable to understand or make sense of.