The trip to Icicle had been... long, to say the least. From Midgar to Junon by chopper. From Junon up farther North still, this time by boat. And then, once they'd landed on the Icicle continent, there were transport trucks to cover the rest of the way. Somewhere in there, Zack had been briefed on the mission. Something or other to do with AVALANCHE, but they weren't really his specialty, and he was way too excited about the fact that he even had work again to pay too much attention.
The trucks had stopped at the rendezvous point, where he was supposed to meet up with a couple more members of SOLDIER and a Turk, and there they waited.
... And waited.
And waited.
Really, with all of this waiting, Zack could hardly be blamed for dozing off the way he did, could he? All he needed was a quick nap, anyhow.
[NFB for distance, yay! For those NPCs that he is waiting for, wheee! This is all going to be ganked pretty solidly from some inconsequential canon bits, a-la
Before Crisis, because I like keeping up on these things like a weirdo. Warning, for impending NPC Death also.]