Coming from upstate NY (I just moved to the Bay Area in Cali in Nov. woohoo!! :hippy::smokin where it routinely gets well below freezing. I just kept the lights on during the coldest hours of the day (typically the night but not always) and that worked well for me. The plants will respond to the cold temps differently depending on the genetics in that particular plant. Plants of the correct genetic makeup will turn purple in response to cold temps but some also turn red, orange or gold. It's just like the leaves turning colors in the fall. Different trees turn different colors. If you're lucky enough to have the right genes you'll get purple leaves and/or flowers.

If you are growing a sativa based strain then cold temps may adversely effect them, although I personally have never had a problem (then again I never had my grow down to the 20's either). The colder temps will cause the plants to ripen faster as well since the plant it trying to reproduce before it dies. This may also cause a lower overall yield if it is in the 20's at night for the majority of the flowering period as was suggested plant growth slows significantly below 50F. Turning off the ventilation it not a very good solution because it decreases airflow and that is a limiting factor in plant production (food, light and water being the others). Without good airflow your plants will not get enough CO2 around them and this will also effect over all yields. I would agree with getting space heater in the area of your tent (not inside it unless absolutely no other option as this may dry the air too much). There were a few other really great suggestions (like getting a controller to turn the ventilation on and off to maintain a set temp for one). Good luck and I hope they don't freeze on you.

Blessed be~