I would buy a dog sized heated bed. I bet they will sleep in it if it is turned up to the highest temperature. Do the beds even have temperatures?

I have a heating pad that I use if my back is starting to ACT UP and when I use it - and roll off it - guess which two kitties are asleep on it in the morning? Yep. Graemer and Miley. They love it. I think the temperature is too high for them, even on low. They also love my heated mattress pad - like a heating blanket except that it goes under the sheets. It's wonderful and the cats will sleep for hours longer if I have it on! And they will also not need to sleep on me the entire night.

I too suggest you either buy a bigger bed or sleep on the couch, although they want YOU so no matter when you sleep unless you lure them away with a warmer object, they will consider you their heat source! (And loving MOM!)