From Longman Dictionary of Contemporary Englishgo/be out like a lightgo/be out like a lightinformalSLEEP to go to sleep very quickly because you are very tired I went straight to bed and went out like a light. → light
Examples from the Corpus
go/be out like a light• A minute later he went out like a light.• Something hit me on the back of the head, here, and I went out like a light.• I went out like a light.• Either it was the brandy or it was the heat, but she went out like a light.• She was out like a light, as soon as we put her in bed. 