You could make something up using those ultra-flat transducers you get for motorbike helmets, and putting them into something like a beanie (or a sweatband?) to hold them in place.

Burton do snowboarder beanies with pads a bit like this built in, but I've no idea how comfortable they'd be if you put your head on a pillow. Not nearly gnarly enough as a test scenario, unless your bed was going down a mountain at the time, probably.

Hugo