Arthritic cats

This cold weather combined with our poor finances has our house a wee bit on the cold side lately. Both of my 20 year old cats have arthritis so they have been wandering around the house squawking quite a bit, even after I go to bed and before I get up in the morning. I know they are old and the cold doesn't feel good, but man, I'm tired! Then I thought about the hot water bottle. It last several hours and once they go to sleep they stay asleep even when the water cools. Of course, I'm also keeping a space heater in the bathroom on very low heat.
Don't they look comfy?! I wish I could relax that well!