how cold is too cold??

Hi dudes...i have a 2 room setup goin..well, i have 2 rooms ready and some babies veggin under a grolux fluro...my room get cold at night...i measured it at 12 deg (celsius) the otyher night...so i have bought a radiator style heater and got it on low in the room all night...this brings the temp up to around 18 deg (celsious)..is this still too cold do ya think??i am usein the fluro to save electricity b4 i put em in the flower room under the 600 watt hps...but if i have to heat the room useing 400 watt heater i might as well use a 400 metla hallide to veg em up??what do u guys reckon??

 

the grow guides say 40F

 

Think i read somewhere its better to have night time temperatures within 10 degrees of you daytime temperature. I.e daytime 78 degrees nightime 68 degrees. but mine often gets 15 degrees colder at night and makes no difference. try and keep it above 60 degrees. This is farenheight.

 

OH! I thought he was asking about extremes. DUHH! Oh by the way the other extreme is 120F