[out like a light] {adj. phr.}, {informal}
1. Fast asleep; to sleep very quickly.
* /Tom got so much fresh air and exercise that he went out like a light as soon as he lay down./* /As soon as the lights were turned off, Johnny was out like a light./
2. In a faint; unconscious.
* /Johnny was hit by a ball and went out like a light./* /After she read that Tom had married another girl Jean was out like a light for several minutes./
Compare: OUT COLD.