Checked the temp this morning when the light had been off all night and it was dangerously low for a betta - 74F. I picked up a mini-heater today when we were at the LFS getting tropicals.

I added a stick-on thermometer to the outside of the aquarium, and cross-checked it with a floating thermometer that I later removed and put in the goldfish aquarium. Temps with the light on seem to be pretty good.