Bucky is fascinated by it. He has two rest modes himself: uneasy dozing and total blank unconsciousness. Sam does both of those but also all the shades in between: dreaming, muttering, squirming, snoring, endless minute variations on the theme of vulnerable oblivion.
Sam sleeps where Bucky can see him.
Sam sleeps in Bucky's bed, in fact, or with Bucky in his, or in cars Bucky drives, or leaning on Bucky's shoulder in the back of a Quinjet. It is clearly evident that Sam regards Bucky's presence not only as not an obstacle to sleeping, but an aid to it.
Sam sleeps while Bucky keeps watch over him.
Bucky guards Sam while he sleeps, and guards Sam's sleep. He wakes Sam when nightmares or danger threaten, but otherwise Sam's sleep is to be protected as much as Sam himself. Unmodified humans need their rest, and Bucky is greedy for the silent demonstration of trust that a sleeping Sam embodies.
Just once, though. When they're at home, and safe, and all is quiet.
Bucky leans closer and closer while Sam sleeps, until he can brush a kiss against Sam's lips. Once, twice, Sam sleeps through even this intimate touch, but on the third achingly gentle pass he makes a sound, tilting his head to press his lips to Bucky's, warm and welcoming even before he's fully conscious.
Bucky pulls back to watch Sam open his eyes. He recognizes Bucky at once, his full lips stretching into a smile. "Miss me, baby?"
"With every bullet so far," Bucky mutters, because it is painfully true and yet every time he says it Sam laughs.
Sam laughs now, sleepy and warm and soft. "You wake me up just to tell me that?"
Bucky shakes his head. "Go back to sleep."
"Gonna need one more kiss first," Sam insists, slinging an arm around Bucky and pulling himself closer.
Bucky makes the kiss a good one, long and lingering and sweetly open-mouthed. "Now go to sleep, Cap."
Sam smiles the same sad smile he always does when Bucky calls him that, blunted now with drowsiness. He cuddles in closer and retaliates with, "Whatever you say, Cap."
Bucky keeps still and watches Sam trust him all the way back down into sleep.