Green’s Dictionary of Slang

1. to fall fast asleep.

1963		K. Waterhouse Jubb (1966) 25: Right [...] flop into me pit, spark out, that’s it for the night.
2001		K. Sampson Outlaws (ms.) 87: I haven’t heard nothing for while. They must’ve sparked out.

2. to knock or be knocked unconscious.

1978		F. Norman Dead Butler Caper 59: It made my head throb and my eyes water, but failed to spark me out.
1989		J. Morton Lowspeak.
2019		Twitter 22 July 🌐 In a very loud voice she said to him ‘heard you sparked out Tommy Robinson in the showers’. A cheer went up from all the other visitors and prisoners.