I have noticed a change in behaviour too! I usually find at least one kitty tucked under the covers. Miley is a heater hog and can be found sitting on the heater top once it has "kicked off" and is just warm, not hot. She will run from parts unknown when she hears the heater fan stop. Juni has not challenged Miley for her spot. Yet. It is big enough for two.

Once everyone's fur grows back in, I think the cats will be warmer too!