Help me my hamster keeps sleeping in his toilet!?

I've got a male syrian hamster that's about 1 year old. Recently, i noticed that he has taken to sleeping in his box of toilet sand, rather than the designated "sleeping" box with his bedding and all, and im not too sure why. He would just take his bedding material from his sleeping box and put it in the box of toilet sand, den proceed to sleeping there. As such, i was wondering if someone could help explain what could be the cause of such behaviour, and perhaps how to create a suitable sleeping box/area for him such that he stops taking to sleeping in his toilet... Thanks