The first time it happens, he was in The Lab. And it was The Lab, capital letters were necessary in describing it in its entirety. He and Bruce's lab. The Lab.
So of course, Bruce was there, working on something or other (Tony hadn't really been paying attention at the time, something to do with the cells of that armored centipede they had fought last week and a microscope). Tony had been working on around thirty-eight hours of non-stop caffeine and doughnuts because sleep was for the weak, and Bruce was around the same time bracket. Tony was yawning into his fist and his head was lolling to the side with the effort to keep it up and his eyes kept blinking sporadically because when he was sleep-deprived he was sleep-deprived, but he was still stubborn enough to keep working through the haze of exhaustion because there was work and the work needed to be done.
That train of thought held up for as long as it was until Bruce pulled away from his microscope, took off his glasses, rubbed at his eyes, and cracked his back. He looked tired, too. The bags under his eyes were darker than usual, and his hair was greasy and unkempt. He'd been down here just as long as Tony, and not seeing the light of day and surviving solely on glazed pastries and caffeine filled mugs for that long wasn't exactly healthy, but y'know. He's Tony Stark, and Tony Stark doesn't let something as trivial as a good night's sleep and three meals a day get in the way of his work. It simply wasn't done.
But Bruce was Bruce and Bruce wasn't Tony Stark, which sucked, but was still true. And Tony liked Bruce just the way he was, giant green rage monster and all, and because Tony liked him he wasn't fond of him being as self-destructive as Tony was in the name of science. Tony could spend a week down in his workshop or The Lab on a maximum of nine hours sleep, running on pure force of will and seven different cups of coffee at a time, and he'd come out of it victorious at creating something but still not completely sure what that something was until his brain had caught up with his body and determined just what, exactly, he had been working on. It was a routine he'd had worked out since he was fourteen, and one Pepper absolutely loathed. Bruce was not allowed to fall into the same routine. It wasn't good form.
So when he'd seen Bruce sit up and crack his back and rub at his eyes, Tony had stopped welding… whatever it was he'd been welding and pushed his goggles onto his forehead to shoot him a look. "You're not gonna pass out on me, are you, Big Guy?" Because that was as sincere and as heartfelt as he ever really got around anyone, despite the overwhelming sense of wrong he felt at trying to keep Bruce down here when he was tired just because he wanted someone other than JARVIS to talk to and it was selfish, and he knew it, and Bruce knew it, so the other scientist took it in stride.
Bruce chuckled slightly, folding his glasses and tucking them into his shirt pocket. "I might if I don't get some sleep soon. I'll just sleep on the couch."
Ah, the couch. It was the comfy, black leather couch they had in The Lab, mostly there so one of them could crash on it if their work was too volatile to leave it entirely alone or if they had honestly just passed out from exhaustion (okay, so the last one was just Tony, but he could not be held accountable for that). Bruce usually slept on it if his legs dragged too much and he couldn't reach the elevator from his station with all the tech and mechanical shit in the way and on the floor, so he'd just collapse face-first onto the couch and start snoring before his head hit the cushion.
It seemed like it was one of those nights (or was it morning? He'd have to ask JARVIS…).
Tony watched Bruce stumble over to the piece of furniture and wow he had not realized just how fucking tired he was until he'd actually stopped working. It was always like that. His exhaustion would creep up on him and as soon as he paused in what he was doing – BAM. Fatigue would roll over him in waves, and he'd usually end up passing out at his desk and drooling all over his schematics. So the exhaustion he felt now was not a good sign, if the sudden blurriness of his vision had anything to do with it.
He saw Bruce topple onto the couch out of the corner of his eye – face-first, just like usual – and faintly heard the soft snores start from where his face was pressed into the fabric. The first thing he thinks is couch, which, in that state of mind, usually means bed, which in turn means sleep, which his body is all about right now. The second thing his brain picks up is Bruce, which means science bro, which his body is also all about right now. It connects Bruce with friend and friend with safe and warm, which – again – his body is telling him are totally good things. The third thing he takes note of is that he's moving and that his feet are actually taking him somewhere, which is kind of surprising because his legs usually go numb when he's this goddamned tired, so he supposes it's his body's way of telling him that it needs to get to the bed to sleep and that Bruce is warm and his body is all about warmth because he's wearing a tank top and believe it or not it's actually kind of chilly in The Lab. A science lab of this caliber cannot simply be "warm" – it's full of state-of-the-art tech and equipment, and having that stuff overheat is dangerous as hell, so chilly it is. But because of that chill, Tony's body has decided to go rogue on his slower-than-usual brain processor and has automatically pinpointed Bruce as a source of warmth, so Bruce it is.
Tony ends up wedging himself in between the back cushions and Bruce's back, slinging an arm over his midsection and nuzzling his nose into the nape of his neck because Bruce is warm and why hadn't he thought of this sooner?
He doesn't even remember telling JARVIS to dim the lights and make sure everything was secure and not about to explode or self-destruct, but apparently the AI got the memo anyway, because the lights did dim and the constant humming in The Lab wasn't nearly as loud as before. It was a blessed relief. He clocked out a few seconds later.