From the spell description:
Sleep (…) Creatures within 20 feet of a point you choose within range are affected in ascending order of their current hit points (ignoring unconscious creatures). Starting with the creature that has the lowest current hit points,(…) Subtract each creature’s hit points from the total before moving on to the creature with the next lowest hit points. A creature’s hit points must be equal to or less than the remaining total for that creature to be affected. Undead and creatures immune to being charmed aren’t affected by this spell. (PH p.276)
Now, to build the ascending order of their current hitpoints, the spell instructs to count everyone in area of effect, but only ignore unconscous creatures. Then later on it says that Undead and creatures immune to being charmed aren’t affected.
Are those two latter kinds of creatures excluded from the roll of current HP by default? Following the “if they were it would say so” philosophy of Sage Advice, it doesn’t seem so to me.