warmth problems

this is my problem guys
atm the cfls are running 24/0 but when i switch to 18/6 the problem is my room is quite cold about 15-16 degrees c and during the dark time i am thinking that the plant will get cold how can i keep it warm without having to keep the lights on.
i was thinking use a heater in my room so rises the temp of room abit but not sure
any help is appreciated

15 - 16C (60-62F) is cool but not that bad.

My room drops to 55F (12C) and so far they are showing no sign of damage from the cold.

I have asked a few people on the board, most feel this will not be an issue, other than the buds may turn a bit purple

cold will slow growth but won't stop it. they will turn purple if there is a 20 degree diff from light to dark you should be okay just don't let it go under 50 degrees F

my shit is 78-80 light on.. then 60 with lights off