go out like a light
To fall asleep quickly. I didn't even hear you come in last night—I was so tired that I went out like a light as soon as my head hit the pillow.
go out like a light
fall asleep or lose consciousness suddenly. informalbe/go out like a ˈlight
(informal) fall asleep very quickly or suddenly lose consciousness: She went out like a light after an exhausting day at work.One minute she was talking and laughing and the next minute she was out like a light. It was very frightening.