Be/go out like a light
Definition 1 of 2
Pro Tip 1/3Quick Sleep
Use it when describing someone falling asleep very fast, often from exhaustion. 
After the triathlon, she went out like a light.
Pro Tip 2/3Colloquial Speech
It’s a casual expression, more suitable for informal conversations. 
At sleepovers, my brother always goes out like a light.
Pro Tip 3/3Not Literal
The phrase is figurative, comparing quick sleep to turning off a light. 
During the flight, most passengers were out like a light.