bah... my room gets down to 50 at night... not ideal but I don't find I'm having any serious issues (when we grow outside we only worry about freezing temperatures killing plants, so...)sativas are built for heat since the originate in the africa region and hotter climates and heavy indicas strains can take a bit of cold i wouldnt got below 60s at nite though too cold and you kill the plants
Keep in mind that, photosynthesis stops at 55 degrees f. If I could make a suggestion, get a small heater with a thermostat. I purchased one at lowes for 20 bucks with a thermostat and it has a fan built in to circulate air.