Definition of "lights out"

There had been one almighty BANG and then it had been lights out.

2007, Kathryn Deans, Glow

It's lights out as Bruno is whacked on the back of his head with a black jack.

2012, Anthony C. TriparI, Butchie, page 112

Instant nausea, impaired vision, and the feeling of impending death, all prior to lights out.

2012, Jeni Decker, I Wish I Were Engulfed in Flames