Generally they just don't really need alot of exercise. Hunting for food would be plenty of exercise for the gecko. You have to remember the gecko and the rodent are totally different in that the rodent is warm blooded and eats alot of nuts and seeds and has plenty of energy to burn off any time of day or night. And the gecko a reptile is cold blooded it needs to rest in the warm to get its body at the right temperature to be able to move around and hunt for its food. It really does not have the same need for exercise as a mammal.