55 is as low as they should go, and even that's pushing it, I have 1 right now that likes the 60's at night, and 3 that don't care; they're all 30 days from harvest. Get a space heater and run it at night(it'll take a few days to get it set right, don't put it on a timer! thats a fire hazard!), then during the day let the lights bring the temps up gradually (turn off the heater in the day to save $$). I'd try to keep it at 60-65 at night with daytime temps from 75-80.