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.
For baseline heating, to get it to the bottom of the range I want for my tortoise, I use seedling mats and ceramic heat elements (CHEs). The seedling mats raised the ambient temp by a 20-30°F, are waterproof and tough, and are relatively cheap (so I have an extra or two laying around, just in case). The CHEs produce a focused area of more intense warmth under them, but with a properly sealed enclosure (which especially with young tortoises is key for holding in heat and humidity) it will warm the entire enclosure, not just a basking area.
I run every heat source I use for my tortoises through a thermostat like the one above. It works by allowing you to set the desired heat range and placing a sensor in the area you're heating; if the sensor indicates it's too cold, the heat element comes on, when the desired heat is reached, the current turns off.
For fine-tuning, or daily checks of how the system is working, I use a laser thermometer gun to spot-check different corners of the enclosure to see how the heating is working... adjust the sensor and thermostat as needed to get the right temperatures for your tortoise.
Once the baseline heating is taken care of, I like to provide an upswing in temperatures during the day, as well as a basking spot for those tortoises that like it; I do this with a mercury vapor bulb, which also provides some needed UV radiation.
I run the light through a timer, starting with a 12:12 light schedule and adjusting from there... I often put in dark spells during the day to allow for cloudy periods or what have you (and also to prevent the temps from rising too much.
I vary the intensity of the bulb I use based on the tortoise I'm planning for: forest tortoises need less intense light and more "shade" time (I use 25 or 50 watt bulbs for my first tortoises), while grassland and desert tortoises (like my Russians) benefit from more intense light/heat as is provided by 75 or 100 watt bulbs.
Getting a good light fixture is important (and not much more money). A bulb with a ceramic fixture that you screw the bulb or CHE into will last longer. The clamps on many of these light fixtures are suspect, weak, or crap-out over time... you can prepare for this by fixing them in place with some form of backup in case the clamp fails (I use a carabiner of zip-tie).
All of this may sound extensive and expensive, but you can get everything I've discussed above for a tortoise enclosure for around $100 dollars, and
KNOW that your tortoise is getting the appropriate heat (and light) it needs, even when you're away.
