Rule #1 - Wash

All you have to remember is Rule #1: When in doubt, Wash. - Paul Gallico

Cats, when not sleeping or practicing the oldest cat trick - misdirection - are probably washing. They must awake from sleep feeling dusty or dingy because within minutes (it seems) they begin the first washing ritual of the day. Somehow the act of sleep must allow dust or dirt to settle on them unknowingly. And when they discover this feline travesty, they naturally remedy it post haste! Though not completely sure, I'm pretty certain that they clean areas where its not possible for gravity to be responsible for placing dust or anything for that matter. But to a cat, the cause of problem is purely secondary, with the primary concern being its removal. So, they quietly (most of the time) set about they business of licking until that urge disappears.

Its interesting to watch a cat go about its daily rounds. Periodically, it will just stop in mid-stride just to give a few well placed licks before it can continue. Does it 'feel' the dust on itself or just how does it know? No matter, the cat knows and that is enough.