If you can find one, look into an oil filled electric heater. They give off a much more even heat and the decent ones have thermostats and timers on them.
cold can cause anything from just a tiny bit of stress which is hardly even noticable.. to completely freezing the leaves off your plants and basically wreaking havoc by killing the cells of the leaves which gather light and produce food via photosynthesis.. also if it gets cold youll notice your plant start to turn purple.. buds, leaves, stems they all turn purple if it gets cold enough.. some strains more than others but people have been known to basically force plants to turn purple by intentionally making it cold at night.. like i say your plants can handle alot and they will make it thru some cold weather but this is not the ideal way to grow nice big healthy mama buds
generally the first thing you will notice if the plant is vegging, growth will slow down until its almost stopped completely and it will just not seem like it has done much compared to when its warmer.. thats too cold, also leaves curling down or getting dry and dehydrated is usually not ideal, although the plant will survive if you keep a close eye and dont let anything too crazy happen to it