Hi Robert,
A preliminary search (I do work) has yielded anecdotal confirmation for the loose blanket hypothesis. I stayed away from quack sites, but who knows who's at the other end. Nevertheless, this concept is mentioned everywhere leg cramps are discussed, so while I wait for a visit to the doctor, I would certainly refrain from tucking in. It seems to be more about loose blankets at the foot of the bed, not so much having the leg itself untucked, so that could be good news.
* On more reliable sites like the Mayo Clinic's they mention blood pressure medication as a very common cause, so you're at high risk to start with.
I would make sure I have all the vitamins and minerals I need, KEEP HYDRATED, and loosen my feet.