Did European artists always sleep with their models?

Not all artists had sleeping partners as models. 

However, there is a nude woman ' Sakia' painted by Rembrandt. He definitely slept with her , as it was his wife.  

yes, maybe...after industrial revolution

If they did, there wasn't much sleeping going on.

No, they didn't always do that.

Was it a difficult task?

Yes, there were no exceptions to this ever.