I the changing of light works.. You can also give them butterflys it helps to get them active. They'll start perking up.. You can also double the light they are getting by putting them on a live plant/tree near the window. Or even place them outside where you can keep trrack of them..
Feeding them butterflies is not a great idea. Some are quite bad tasting or even toxic. Plus, you won't know what specific species he has there in Bulgaria. They may be coated with pesticides too.