But that happens to me sometimes.
One time I came home from work in the evening and I was really really really tired. So I took a nap and I woke up twice throughout the nap thinking it was the next day. It was kinda freaky...
Something made a loud sound (like someone dropping something) that woke you up, but since you weren't fully aware of your surroundings until well after the sound occurred, you didn't know what it was.