In part 2, chapter 9, (pg 179-180) of Orwell's "1984," Winston awakens from what feels like a long sleep. He was exhausted and now feels rested. He looks at the clock and reads "twenty-thirty" Julia awakens and they hear the woman singing in the court-yard. It is puzzling that the fire is out, there is no coffee, and the air has turned cold. Something is off, but again, because of the clock Winston and Julia don't realize that they have over-slept. Winston does notice that the sun is no longer shining in the courtyard, but he tells himself it must have dropped below the buildings. He also notices that the flagstone in the court yard are wet. He has a feeling that the time is not right, but the clock says twenty-thirty, so he simply shrugs off the feeling. Everything seems normal until the picture on the wall crashes to the floor and the tele-screen is reveled.
Because they were tired from last night having sex.