h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Your duvet is full of bugs... dust mites lurk in all of your duvets. The movement of the duvet maybe the little bugs acting in cohorts, rubbing your skin off to obtain their next meal. Or they could just be having a party in one corner. See Asthma for more info on mites: http://www.h2g2.com/A70967

But then, so are your blankets and any other type of bed-linen. Throw it over the balcony as they do anywhere it's sunny enough and have it cleaned regularly - easier if it's synthetic than feathers and hopefully you're not made worse by whatever's used to clean it.