See this cartoon at 0:10.
A bulldog was asleep.
While cat Tom walked on tiptoe by the bulldog, the bulldog happened to grab cat Tom by the leg by accident "in unconsciousness".
I don't know which word should I use, "in unconsciousness" or "in his dream" or "on instinctive reaction" or something.
How do we express someone does something unconsciously while he/she is asleep?
Best Answer
The medical term for this is parasomnia. If you suffer from parasomnia, you might move around, talk, or do unusual things during sleep. Other people might think you’re awake, but you’re actually unconscious.
The term used by most lay people is in [his] sleep. As this NGram Graph shows, the expression is reasonably common. Here is a typical example: