의미가 다른 문장 찾기, 5번은 왜 다른가요?

다음 문장 중 의미가 다른 것은 5번이라고 하는데, 왜 그런지 설명 부탁드려요^^
1. On going to bed, I fell asleep.
2. As soon as I went to bed, I fell asleep.
3. The moment I went to bed, I fell asleep.
4. I had hardly gone to bed before I fell asleep.
5. No sooner had I fallen asleep than I went to bed.