Steve's a protector, always has been. It's only natural for him to want to hold Tony when they're sleeping, to curve his larger body over Tony's so Steve can shield him from anything that would hurt him, even when they're in the Tower and perfectly safe. That's Steve's immediate instinct, and he worries that Tony will balk at such protectiveness, but Tony surprises him.
Steve assumed Tony would want to wrap Steve in his embrace when they slept in an effort to regain some control, to not feel so vulnerable after Steve worshipped his body and led their lovemaking, but Tony doesn't do that. Instead, Tony easily turns on his side, putting his back to Steve, and reaches behind to grab Steve’s hand, tugging. Silent, Steve scoots closer, fitting his body along the curve of Tony’s, and slowly puts his arm around Tony, his fingers splayed out low on Tony’s stomach.
“Is this okay?” Steve whispers, unsure. He regrets breaking the peaceful silence, but he needs to make sure this is really want Tony wants, that he'll be comfortable sleeping this way. "We can switch."
“It’s fine,” Tony says, sounding a bit amused, and he shocks Steve by covering Steve’s hand with his own and rubbing Steve’s fingers. “You worry too much.”
Steve wants to say You don’t worry enough, not about yourself, but he holds the words back; they'd only ruin this moment, and they don't belong here anyway. So Steve just settles against Tony with his nose pressed into Tony’s neck, the smell of Tony making something that's coiled tight inside Steve's chest relax. His eyes close as Steve tangles their fingers, and Steve feels more content lying here with Tony in the aftermath of their lovemaking than he can remember feeling since awaking in the future.
This is how Steve hopes they sleep every night: so close and tight that Steve doesn’t think anything can separate them, nothing between them but love and companionship. As much as Steve loves holding Tony this way, he's not picky: what's most important to him is feeling Tony near, making sure Tony’s safe, whatever position they end up falling asleep in. If Steve can have that, he'll be content.
"I love you." Steve sleepily breathes the words over Tony's ear, half sure that Tony's asleep already, but still needing to say the words aloud.
Tony stirs and he chuckles a bit, but he briefly tightens his grip on Steve's fingers. "Of course you do," he says.
The words are quiet, but still so full of Tony's normal self-assured arrogance that Steve can't feel anything but fond affection at hearing them – he wouldn’t change Tony for anything.
"And," Tony says, this time bringing Steve's hand up higher on his chest, laying it over his heart, "maybe I love you, too."
Steve's fingers splay over the warm skin and he's not embarrassed by the sudden prick of tears – he just falls deeper in love with Tony, deeper than he thought he could.
"Now go to sleep, Cap; you need your beauty sleep," Tony says, and Steve just smiles, brushing a feather-light kiss over the back of Tony's neck.
Tony's heart pounds under his hand. "Yes, Tony."