temp. whats too low?, for dark period?.

im growing outside in the garage in a closet. im having the 12/12, lite cycle. on 9am. to 9pm. just incase anything happenes in the nite?.
in the daytime the temp is about 80 with out a fan . but i still use one. but on a nite when the lites are off, its getting just under 40deg.. is this bad?.
and is it why?, my plants seem about 2 or 3 week behind?growing good but slow.for some reason. im feeding on biobuzz and am flowering under 400hps. im growing in soil.i had 5 biggish plants under 1 400hps. but now only one 2foo, and 3 small ones.cos when i grow bigish they seems to be lots of waste, with just 1 lite.
were am i going wrong is it the temp thing?.

plants don't like it lower than 60 degrees so id get a heater for them a low energy one. and growing in a garage is risky hope you have'nt told anyone or you could be looking at a raid from theives.

Good info from Widowman ,
Sure plants will survive as low as 40 deg. , but from optimum growth / thrive 60deg. low , 82deg. or so as a high .
Peace
The Ghoul

how much will it put em back at 40. the widowman, have you read my wtf post i posted yesterday.