My lot have hay on their floor (paper underneath) and carpet on their 'sleeping shelf' (why they won't sleep on the hay is beyond me). At night, if it's cold, I put in their thermo vetbed for them to sleep on and a heatpad. If it is warm, I just put a bit of duvet in, I like to think they're sleeping on something soft.
Steph- I put mats in the run to stop cold bottoms too!