My awesome rat vet was discussing things we can change in our rats' cage to make them more comfortable now that they're older (and Milosh has pneumonia). He suggests using a heat lamp or heat radiating panels on part of their cage. I got them a low wattage night heat bulb so the light is low and the top shelf of their cage is around 75 degrees now. They will not leave the shelf. I feel bad that they didn't have a warmer spot all along. Just thought I would pass the idea along because my baby-guys are LOVING this! You can't see Milosh behind Cato, but his head is poking out the back of their hut and he's sleeping.