Well, ya kinda said it yourself- if you used that knowledge, it would have been metagaming.
I'd say that little circle of vision exists to provide players who are asleep for an extended time (surgery, etc) a view into the game, so they don't get bored. A nice medium between 'your character is knocked out so you can't see anything no matter what' and letting the whole screen be visible. There's always quality control, bringing the hammer down on people who metagame stuff they see while they were supposed to be knocked out, if that's the problem, and besides, who's going to believe a dude who springs up out of a sound sleep and yells "HEY, I SAW (XYZ) JUST NOW!"
Is your concern the immersion quality? Hearing stuff and seeing that little circle? Perhaps think of it as the way you slip in and out of sleep, and catch little glimpses of what's around you, but don't really recall it afterwards? Really, I just see it as being for the player's sake, but that could be your IC inbetween, or something of the sort