They do like round things to curl up in! charlie will also head for something round
Coconut shell houses are popular round things to curl up in too - you could maybe put something like that or a mug even, in a darker part of the cage under the shelf.
I hate to say this but the cage does look quite small - they need a fair bit of floor space and overhead cover in a cage, like a shelf sometimes. A smaller cage can be ok if your hammy free roams or has plenty of out of cage time, but the minimum recommended floorspace size is about 70cm by 40cm for a dwarf hammy x