People wake up when being knocked out

[The_Silver_Nuke]

I'm trying to kill this one guy in his sleep, but I haven't unlocked the skill to kill with a dagger yet, so first I'm attempting to knock him out. When I knock him out, another person wakes up immediately and revives him, and that continues endlessly. How the heck do they hear me knocking him out when I muffle his scream and such?

Is there any way to knock him out stealthily without someone coming and intervening?