The first time I heard someone use the phrase "I passed out" meaning "I fell asleep from exhaustion" was back in the 80s, when a New York City friend of Bunnicula's used it that way.
I had always assumed that "to pass out" meant "to faint" or "to lose consciousness suddenly, wherein one falls to the floor". That's how I had always heard it up until that time.
When did the meaning "I fell asleep" come about for "I passed out"??? Weird....I always get this mental image of a person being so exhausted that s/he suddenly falls to the floor in a heap and spends the rest of the night there until s/he "comes to."