This is very individual. Our degus slept in the nest box in the beginning, then I gave them few levels into the cage and they started to sleep on the most top level. Then they slept on one of the top branches for few weeks and another time they slept often on the bottom of the cage.
Most often they like to sleep in the top part of the cage, but if they are not used to the place where the cage is or if the place is too eventful, they sleep in the corner or in the nest box. Otherwise they sleep in the nest box only when they have babies.