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.
Best Answer
Looking at those two fragments:
it seems like "being affected" means having your hp deducted from the roll and being subjected to sleep, so therefore this:
would mean to me that these creatures are not subject to the spell at all, just like unconscious ones. Also note it says "aren't affected by this spell", and not "aren't affected by the sleep effect" or something similar.