It's a good thing you were wearing your seatbelt. Being asleep also helps reduce injuries. Your muscles are all relaxed and don't tense up before the crash. That, unfortunately, is why drunk drivers usually survive a crash even though the person they hit, don't.
I've never crashed from sleeping, but those warning grooves they put on the side of the road in Wisconsin sure have saved my life.
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351W 89 Mustang GT Convertible