According to the 3.5 PHB (p. 309), a helpless (e.g., sleeping) target is treated as having Dex 0, and has a "-5 modifier". On p. 153, it further explains that a helpless defender can't use any Dexterity bonus "to AC. In fact, his Dexterity score is treated as if it were 0 and his Dexterity modifier to AC as if it were -5." (Emphasis added.) It also states that a coup de grace automatically hits and does critical damage.
What I can't seem to find is any discussion of whether a sleeping or otherwise helpless target gets a Reflex saving throw. I'm particularly interested in whether a helpless target gets a Reflex save vs. dragon breath weapons.
It seems to me that a helpless, particularly sleeping, target won't even know it's under attack, and thus won't be able to "escape by moving quickly" when dragon breath, a spell attack or the like comes at it.
How do the rules deal with this situation? If they don't address it, how can it best be resolved?
Best Answer
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.
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.