I used 1 of these 2 years ago during the winter months.
Sunbeam Ceramic Heater: Heating, Cooling, & Air Quality : Walmart.com
I had it hooked up to a timer it would go on when the
grow lights turned off. It kept the grow area warm at the same temp as when the
lights were on. If I hadn't used it my plants would've died from the drastic temp drop.