Wiktionary
out like a light
a. (context simile colloquial English) asleep or unconscious, particularly if this has occurred suddenly and the sleep is deep.
Wikipedia
Out Like a Light
Out Like a Light is a live album by jazz musician John Scofield.
Usage examples of "out like a light".
Carl shuffled off into a corner, wrapped himself in his sleeping bag, and was out like a light, much to the envy of the others.
Delicate she was to look upon as any princess who had ever adorned a court, delicate and fresh, rich-voiced and young, but here was the rare woman flashing out like a light over stormy seas, the spirit of her and her courage!
But, if Marga doesn't know, I surely do not, as I got banged on the head and went out like a light.