How cold is too cold?

I have recently planted tomato seeds in a small green house incubator outside. The weather for the most part has been pretty fair (mid 70's) and the lowest it has dropped is about 53 degrees Fahrenheit. The seeds haven't even sprouted yet and I'm worried that it has been too cold for them. How cold can they get before dying, or get to a point where they won't grow? I'm a absolute beginner and any help is immensely appreciated.