The rule is clear. The hit that drops you to 0 is the one that does the real damage. Until then you are completely unscathed or suffer minor bruises/scratches. There are exception, but those are for things like poison stingers which do have to do minor damage to you before you would normally take scratches and small cuts.
"When your current hit point total is half or more of your hit point maximum, you typically show no signs of injury. When you drop below half your hit point maximum, you show signs of wear, such as cuts and bruises. An attack that reduces you to 0 hit points strikes you directly, leaving a bleeding injury or other trauma, or it simply knocks you unconscious"
There's your citation and there is no new rule for it in 5.5e, so that rule still stands. If you are sleeping and are stabbed in the heart, you are dropped to 0 or else you weren't stabbed in the heart.