Are your two hides identical? She could be favouring one hide over the other.
Is your tank anywhere near a speaker/clock or anything else that might vibrate? That could be scaring her away to the other side of the tank.
It's also possible that when you go to bed she's sleeping on her cold side, and she wakes up, warms up, does her secret snakey stuff while you're asleep and then she goes back for a sleep before you wake up.

You might also want to check your accurite - I know it will give you the min/max temps within the past few hours (mine does anyway... it has a MIN/MAX button) and you could check if your heat is dropping through the night.