I'm not sure what the extreme lows can be, but if you can keep them around 60 I can tell you that you have NOT to worry. Just so you know, I was WAY under that in my last grow. I don't recall the exact temps, but I'm sure it was below 50. If you can, I would try to keep them around 65 while the light is off. It's starting to get nippy around me as well. Even though the nights are starting to drop, I still take my pots out for all day sun