Depends on what you define as "really cold". Even if you normally keep a gradient of 70-85, it won't hurt them one bit to get down to around 60 for a few days/week or two. Don't give them a big meal before you leave if that's going to be the case though. It's hard to digest at the cooler temps. If you can't use a timer, but need heat, use a red bulb or undertank heater. They seem to get very annoyed with 24 hours of bright light.