As soon as I decide to take a nap:
10: They start mowing the lawn in the common gardens behind our house.
9: The three-legged cocker spaniel next door decides to have a bark-fest in the back-yard.
8: The phone rings.
7: My cell-phone rings.
6: The door-bell rings.
5. Allergy season gives me raging headaches whenever I'm horizontal.
4. If the dog is in the room, she snores.
3. She also has bad gas.
2. My son's asthma has in no way limited his lung capacity or ability to thunder down the tile stairs yelling "Moooooooooooooooom" for whatever his latest need may be (fix the TV, kill the fly/bee/bug that is annoying him, getting him a drink/snack, asking to play on a computer etc.)
And the top reason I cannot take a nap...
1. My husband comes in just when I am either about to fall asleep or have just done so, and starts shaking the bed while he laughs about his latest "joke".