There are lots of considerations to keep in mind in caring for your tortoise, or creep, perhaps none more important (or at least pressing) than providing the right amount of warmth for your cold-blooded friend.
If too cold, tortoises tend to become torpid and inactive, which is okay if they're one of the kinds that brumates (a reptile equivalent of hibernation) and that's what you're doing with them. If the cold temperatures are not by design, the inactivity can result in poor digestion, as their guts work best at processing food at 80°F (about 27°C) and above.
If too hot, tortoises can dehydrate and even get burned and damaged shells from over-warm heat sources.
Obviously, providing heat in the proper range for your tortoise is important... luckily, it's pretty easy too.