Heating issue for corn snake please help, best answer chosen?

Hi i have a problem with the heat setup for my corn snake. I use two lamps, one infrared and one regular basking bulb. during the day i put both lights on but at night i only put on the infrared. on my warm side, the temperature during the day is 80-82 degrees but on the cold side it is only about 67-68 degrees. Why is the temperature gradient so large, is there anyway i can insulate the tank to prevent heat from being loss. Also at night, the temperature drops all the way down to 60 degrees is this too low for a corn snake? Thanks for all your help.