Yes it's very common especially in light sleep as you are still drifting into deep sleep (REM sleep), and are still physically aware of where you are.
You begin dreaming as soon as you start drifting off to sleep and if anything happens to make you jump in your dream, your body will respond to it as if it were real, making you flinch.
Eventually when you fall into deep sleep you don't normally flinch or jump up during dreams.
so they want to warn their enemy or they want to be safe
No, It's very normal, although I do not know why they do it.
Flinch
According to IPA, the phonetic spelling of flinch is /flɪntʃ/ In spelled pronunciation, the word is pronounced /flinch/.
The inside fastball caused the batter to flinch.
Every time I watch a 3D movie, I flinch.
Learning to Flinch was created on 1992-06-24.
The football came towards me so fast, it made me flinch.
Everyone flinched as the shells exploded all around us. Try not to flinch when the ball is thrown at you. The serial killer didn't even flinch as the guilty verdict was read. The brave child didn't even flinch as the nurse administered the flu vaccine.
In some dialects, shudder and shutter are pronounced the same.
flinches
recoil