It doesn't mean anything, as far as I know. 'Bone sleep' seems to be just two random words put together, and it's not worth worrying about.
What is worth thinking about, though, is a little bit of very basic grammar. Do you remember your elementary English lessons, when you learnt how to ask questions in the present simple, with the auxiliary verb 'do'? 'Where do you live?' 'What does he do?' and so on? So your question should be this:
'What does 'bone sleep' mean?