you may just bruise in your sleep, if you move around alot!!
You could be aneamic, more iron in your diet may help. I bruise incredibley easily, no specific reason, I just do.
take an iron supplement or eat more red meat, at least four times a week to keep your iron levels up. i also bruise very easily and this is the advice a nurse offered to me, so i would give it a shot
Take extra Vitamin C. Get the kind with Rosehips.