It was a tacit rule on field missions that when you heard one of your comrades' sleeping bag rustle slightly during the night, you did not ask them what was going on.
For one thing, it wasn't hard to guess. Naruto did his fair share of rustling after all -- and maybe a little more than that -- and even if he hadn't, the smell would have told him everything he needed to know.
For another thing, while seeing the bastard blush and scowl was entertaining indeed, Sasuke hit harder when he was embarrassed.
Of course, usually the rustling came from Sasuke's sleeping bag or his own. He assumed that Kakashi-sensei did... whatever he wanted to do during the day, or was just amazingly stealthy. Anyway, it didn't matter. It was a guy thing, and they weren't supposed to talk about it... unless Sasuke actually made noises, which didn't happen often -- but when he did Naruto felt a sort of obligation to tease him the morning after, to get revenge on the way Sasuke's fun time had intruded on his own personal moments.
Then of course Sasuke got pissed and they beat each other up, which was... not really a problem anyway.
But it was a guy thing.
Naruto wasn't sure what the protocol was when it was the girl of the group who started squirming in her sleeping bag.
There was no privacy during missions, especially not the ones that required them to sleep in arm's reach of each other, but they all could pretend. So at first, he tried to tell himself she was just having a bad dream.
Except that since he didn't dare turn on his side away from her -- it would have made noise, and then she would have known he was awake -- he could see the way her sleeping bag tented when she folded her knees, and the way the cloth twitched and creased. Then of course he couldn't help but marvel -- girls did that too? -- and wonder how she was doing it -- girls didn't have a penis after all.
It smelled nice. The rhythm, though, apparently needed some work. And it lasted so long, that by the time she let out that high-pitched, strangled little squeal, he had been pushed to drive his nails into his own thighs to keep from going crazy.
He beat his own record that night. Not even twenty seconds. That was a lot more efficient. Girls were so slow.
Pondering that deep mystery of the universe, he fell asleep.