Quoting Anna Aho:" I've heard some horror stories, my step mom was sleeping and her water just broke on her. I'm 1 cm dilated ... [snip!] ... dropped much lower since then and he feels further down. I'm not sure if that has anything to do with being effaced or dilated."
Yes, a freak delivery could happen, but it is very seldom for a first time mom to have a delivery in less 4 hrs. And if you are sleeping and your water breaks, which is pretty rare, you will still have plenty of time. Maybe have a plan of action in case you do go in to labor. If it happens in the middle of the night, then this will happen, if it happens while so and so is at work then you will do this, if it happens on a weekend then this will happen. I think having a plan for various scenarios would help you relax about it.