They will stack for the warmth, to be nearer to the heat, but they're stacking all night? Do you have the lights on all night? (If you're using a red heat light or another form of heat that doesn't give off light, this doesn't apply.) If the lights are on, they should be off so your turtles can sleep (the lights will interfere with their sleep cycle). Lights should be on for an average of 12 hours a day, slightly less in the winter months would be good. Using a timer would be good as well.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-