it's really important to keep the temps up, not just for his comfort but for proper digestion and other internal systems. If he is unable to bask at high temps for several hours (especially before lights out) his food may not be fully digested it then sits and ferments in his cold belly all night, which can lead to health problems.
You should be able to keep the temps up with his lights and you may need to get a ceramic heater (be sure to use in a ceramic socket, not plastic). You may want to drape a thick towel on one or more sides. I keep my house quite cool at night so I cover Darwin and Cyrus' cages with towels/blankets to keep it from getting TOO cold, especially for my little guy.
What are you using to measure his cage temps? If I haven't said it before, the $25 investment in a temp gun (www.tempgun.com) is worth it.

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Chameleon Help & Resource Info
1.0 Nosy Be Panther Chameleon - Cyrus
0.1 Veiled Chameleon - Luna. She's now hanging from her big jungle gym in the sky
1.0 Beardie - Darwin
0.2 felines - Kyndra and Lita
0.1 African Clawed Frog - Skipper
0.1 Mad. Hissers and she's back!
0.1 Chilean Rose Hair Tarantula - Rosa Leigh
0.1 Goliath Bird-Eater Tarantula - Natasha
?.? Pinktoe Tarantula - no name yet