Gneech, that's actually close to my suggestion. The OP discussed how he dropped the hints to the party of the hobgoblins being underpaid and the like, and mentioned here to us that Nezzner would likely want them alive.
If they were sleeping without watches, I would have the sound of movement during the night (as hobgoblins approaching the barracks see the dead and the adventurers and pull back to plan). The PCs would have a chance to hear this, but it wouldn't be great. If they're all sleeping, I'd have them wake up surrounded by hobgoblins (mages especially watched with ready actions for spellcasting – fighter-types would already be unarmed and unarmored) with an officer very interested to have a conversation with them about who they are, what they've done, what they want, and what they can do for their new hobgoblin friends in exchange for getting out alive...
I'm reminded first off of the times as a LARPer that I and others would wake up first thing in the morning in similar circumstances as the story called for it. It wasn't a case of "I'm going to kill you at 6am!" "Fine, then I'm going back to sleep!" as much as what this scenario here presents: an opportunity to play with the world of the game and invert power dynamics based on player choices. To kill off the PCs (especially coup de grace them in their sleep) is the laziest possible option, even if it's the "logical" one. In any good fantasy narrative, sleeping in hobgoblin barracks is just going to get you captured by hobgoblins while sleeping; a nice touch would be, since they killed those other gobbos and are in their bunks, now THEY'RE being conscripted to take their place ("Get up, maggot! Welcome to the Hobgoblin Army, soldier...").