Using of out like a light
Hi! Could anyone tell me the meaning of the idiom "to be out like a light"? Of course I tried to find it by myself but I am not sure that I realize everything right. Some sources explain that this phrase means "to fall asleep quickly", e.g. I was out like a light when I got home after my hard work, other ones explain "to be in a deep sleep or unconscious", e.g. "I haven't heard any sounds around because I was out like a light". Just going to sleep and being already sleeping or unconscious are surely not the same. Dear native speakers, tell me which situations d'you use this phrase in? Give any instances please.

Question

Using of out like a light 
Hi! Could anyone tell me the meaning of the idiom "to be out like a light"?  Of course I tried to find it by myself but I am not sure that I realize everything right. Some sources explain that this phrase means "to fall asleep quickly", e.g. I was out like a light when I got home  after my hard work, other ones explain "to be in a deep sleep or unconscious", e.g. "I haven't heard any sounds around because I was out like a light". Just going to sleep and being already sleeping or unconscious are surely not the same. Dear native speakers, tell me which situations d'you use this phrase in? Give any instances please.

Answer

I would use "out like a light" for the process of going to sleep very or losing consciousness quickly - going to sleep, heavily drinking, being knocked out by a punch.  The image is one of turning off an electric light  - darkness (unconsciousness) is achieved very quickly.
It should not be used for deep sleep - where you have already lost consciousness and can't waken or regain consciousness easily or very fast.
The better phrase in such a situation would be "out cold".
For just sleep, you can say a person in a deep sleep is "sleeping like a log".

Answer

We use it in both situations.

When a light is out, then there is no light.
Think about it!

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