As I was changing my Pacman Frog's water, I noticed that the substrate in the terrarium felt cold. It normally feels about the same as room temperature (which is the temp. of the water I spray him/her with), but I was wondering if this is common when it's near substrate changing time? It's almost been a month (last changed substrate on the 1st of this month), so I'm guessing the temp. drop is due to all the moisture the substrate has collected, but correct me if I'm wrong.

PS: I keep my apartment at 78 degrees during the day, 77 at night, and bump it up to 82 when I'm out of the house. The terrarium humidity stays between 70% and 80%. I also have a small tank heater waiting for winter use, but let me know if you think I should use it now.