You know, the same way we accidentally do it on airplanes. Which, by the way, is a great way to break the ice with a hot girl sitting next to you if you warn her it might happen: "Hey, I'm going to try to sleep. If I fall over on you, wake me. I've been known to do it." Even if that is a lie, you seem courteous and now you have a reason to talk to her once you wake up. Ice broken.