That used to happen with my dad all the time when we were kids. He'd fall asleep on the couch watching Carson and wake up in a startled commotion if we happened to shut the TV off. If you tried to wake him up on your own so he could go to bed, he'd never hear you. Shut the TV off and he was up in an instant.
I always wondered if it had something to do with his WW2 experience. Soldiers (and mothers) train themselves to listen for certain sounds while sleeping and when they don't hear them anymore or the sound changes, get alerted to something being wrong.