http://www.youtube.com/watch?v=qApq5O73piw&feature=related
You are constantly shedding dead skin. When you lie in one
place for 6 hours (aka sleeping), tonnes of dead skin cells will
accumulate, whether you shower once every 20 minutes or once a
day
They aren't on human skin, they are in your bed eating the dead
flakes of skin you've shed there. They then shit, and die, and
since they are so small their shit and dead bodies combines into
dust.
Unfortunately some of us are very allergic to dead dust mites
and their faeces
