if it's that cold in there then i would get a heating blanket and put under them.keeping the roots warm is more important than the plant it's self.anything below 60 and it can effect the plants growth.like said above it can cause stress on the plant and thats not good depending on strain.

the reason why cold slows growth rate is because it slows uptake of nutes and it's break down of the nutes to uptake as food.