They were married, but since the argument they had a few days earlier, they hadn't been talking to each other.
Instead, they were giving each other written notes.
One evening he gave her a paper where it said:
"Wake me up tomorrow morning at 6 am."
The next morning he woke up and saw that it was 9 o'clock.
Naturally he got very angry, but as he turned around he found a note on his pillow saying:
"Wake up, it's 6 o'clock!"