How to insert a skeleton PC into a dream? [on hold]

I want to organize a session during which my players will find themselves in a kind of dreamland where I will be able to present the BBEG and some other story points in a way that does not really affect them. So they go to sleep, "wake up", go in this type of quest to face the BBEG, they feel the difference in power level with respect to them, probably end up dying, but when they die, they wake up dreaming realizing that it is a thing of type "vision of the future".

I think it's an interesting and interesting idea, but I realized that one of my group members is actually a skeleton PC. Technically, the undead do not sleep and now I have no way to include it.

I hurt my brain, but I can not think of an intelligent solution that allows him to participate in the dream / vision. No suggestion?