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.
Has anyone ever dealt with this situation? If so, how do you resolve it? Thanks for any advice.