If your using HID Lamps you should have no problem
with low temps when the lamp is on.
I use an 800w small Oil filled heater to keep my girls warm
at night
I grow in at 4 foot long 4 foot wide and 6.5 foot high space
my low temps are around 60-68 at lights out
and can hit top side of 90f lights on but is generally at 85f
most of the time. all my 8 girls look good.
Id try and not let your temps drop below 60f if you can help it
and iv heard you can grow with temps upto 90f without any
additional Co2.
Good Luck