Barring text to the contrary, I'd say it stays asleep until it makes the save. It's not like normal stimuli are sufficient to wake somebody up- the spell makes people fall asleep in the middle of a raging combat, and you can't wake your partner up just by screaming at him. Stabbing him might be more significant, but if magic's making him stay under when a bugbear marauder is screaming next to him, then I can't see why it won't make him stay under when the marauder bashes his head in.
In a game-balance sense, the average creature only has a 45% chance of falling unconscious for one round, and half that for staying under for two- and that's assuming the spell hits in the first place. And it's a daily spell. I don't see any problem with giving free coups to a party, especially since coups are now only max damage instead of instakills.