[RPG] Can a sleeping creature make a reflex save

This is not strictly RAW, but it is from an article called Rules of the Game: Actions (Part Two) by Skip Williams on the WotC website.

It's usually best to allow... opposed checks to resist being tripped or grappled, unless they're helpless (any ability score reduced to 0, paralyzed, petrified, or unconscious). It's worth noting, however, that even a helpless character can make a Reflex save (albeit with a -5 penalty for an effective Dexterity score of 0). That's because the game generally favors at least some involvement from the player -- and a chance for a miraculous escape -- whenever a character is in danger.

So Jonathan Drain's comment is dead on. The saving throw doesn't map directly to dodging out of the way, it's a metagame mechanic. A good way to think of it is that the "reflex" part of the save is best used after the roll to narrate how the character avoided death.

• A helpless character is paralyzed, held, bound, sleeping, unconscious, or otherwise completely at an opponent's mercy. A helpless target is treated as having a Dexterity of 0 (-5 modifier).

  This applies to all uses of Dex, including reflex saves. So if you can't move, then that really is a huge penalty to dodging fireballs.

• A creature who is flat-footed, or who can't see the attacker, is denied Dex to AC, but I can't find any rule saying that they lose Dex to reflex saves. You're right that this is a bit weird.

• A creature who is flanked doesn't lose Dex at all; the attacker just gets a flat +2 bonus to attack rolls. The fact that this doesn't apply to reflex saves is irrelevant, since you can't flank using anything that doesn't involve an attack roll.