The first time it happened was after a case.

Sherlock had gone almost a week without sleeping or eating, other than the toast John had managed to sneak to him when he'd made himself two pieces instead of one. Sherlock had automatically stolen off John's plate when he put it down within easy reach of his flatmate, to John's gratification.

The sleep was harder to enforce; Sherlock was so manic during his cases that John couldn't really trick him into resting.

For the past six days, John had gotten up in the morning after a fitful few hours to come downstairs and find Sherlock exactly where he'd left him when he'd gone to bed. Sometimes he would be playing his violin frantically, or pacing about the living room in circles, running his fingers through his hair until it looked like a rat's nest. Other times he would be curled up on the sofa, hugging his knees close to himself and rocking back and forth. Or in the kitchen peering into a microscope, or standing over a test tube with a lit flame and the safety goggles that made him look like some kind of mad scientist.

John knew better than to bring it up. Sherlock would always run himself ragged during an interesting case, and then collapse as soon as it was over. The last time John had mentioned to Sherlock that it really wasn't healthy to treat his body that way, they had gotten into a huge row and John had spent the night on Sarah's sofa.

That was ten months ago, and Sarah's sofa was no longer an option, nor anyone else's, for that matter. Although John was certainly charming enough to get women to go on first or second dates, or even in their flat "for a coffee," ultimately the strange relationship he had with Sherlock would put them off. No wonder everyone thought they were a couple. When Sherlock texted John with a new case, John would drop almost anything to come join him. He'd left a date in the middle of dinner before, when Sherlock had sent him a series of texts that had made John worry that he'd been kidnapped. Of course, it was never so dire as Sherlock made it sound. Sherlock was perfectly fine when John came back to Baker Street, although there was a large black mark on the kitchen table and quite a bit of broken glass.

So John had said nothing about Sherlock needing sleep until the seventh day, when Sherlock had leapt off the sofa and texted Lestrade, grinning triumphantly and muttering something about red paint and chickens, and John really couldn't be arsed to get Sherlock to explain it.

"Bed," John declared, pushing Sherlock towards his bedroom.

"I can't," Sherlock mumbled, craning his neck back towards John and digging his heels into the carpet.

John stopped pushing, but his hands stayed on Sherlock's shoulders. "What do you mean, can't?" he growled.

"Experiment on my bed. Need to move it first. And wash the sheets," Sherlock added, hands fluttering about.

Ugh. John wasn't terribly surprised. Sherlock slept on the sofa enough times, and he had no qualms about putting body parts in the fridge or leaving his books and case notes on every available surface. Sherlock was not a man who paid much attention to boundaries. So doing some sort of messy experiment in his bed probably had made perfect sense at the time. John certainly wouldn't have let Sherlock do it in the living room.

"Fine. Use my bed. I'll stay in the living room while you get some rest." John wasn't so exhausted that he couldn't wait until Sherlock was done napping.

Unfortunately, Sherlock looked entirely too delighted at John's suggestion, so John hastily added, "Sleeping only, Sherlock! No moving around the furniture, and no rummaging through my things, and especially no experiments. Okay?" He stepped back and crossed his arms over his chest, eyebrow raised and studying Sherlock sternly.

Sherlock's grin turned into a grimace, and he waved his hand at John dismissively. "Yes, yes, fine. I'll see you in a few hours." He retrieved his pyjamas and dressing gown from his bedroom and tromped sullenly upstairs, slamming John's bedroom door shut as he flounced inside.

John put the kettle on to boil and grabbed his laptop from the kitchen table where Sherlock had been using it. He saw a web page open to a forum post about do-it-yourself roof repair, and John just shook his head in puzzlement and brought his computer over to the living room. It would be a while before Sherlock came back down.

When John came to, the living room was dark, his half drunk cup of tea was stone cold, and his laptop had powered itself off. There was still no sign of Sherlock, and John had underestimated his own need for sleep. Time to kick Sherlock out of bed.

But after John had stumbled up the stairs and peeked into his bedroom, he didn't have the heart to carry out his plan as intended.

Sherlock was sprawled out on the bed, sheets kicked into a tangled mess around his legs, and clutching John's pillow tightly to his chest. Sherlock's trousers and shirt were carefully folded on the floor next to the bed, and John marvelled at how careless Sherlock was about most things when he was so particular about his appearance. So much of Sherlock's outward persona – his apparent grace, his facial expressions, his hair and clothing – was controlled. But there was nothing guarded about him now, as he slept.

John didn't get to see Sherlock like this often. He often waited until John was at work to take naps on the sofa, and John rarely attempted to venture into the man's bedroom. John smiled at the memory of Sherlock's catatonic state after Irene Adler had drugged him. He looked more peaceful now, and there was thankfully less drool.

So instead of shaking Sherlock awake and reclaiming his bed, John decided there was probably enough room for both of them, and slid carefully into the left side of the bed. He stole back a handful of covers, mindful not to tug hard enough to wake Sherlock, but the detective was dead to the world.

When John woke up, he was tangled in a mass of long, warm limbs and dark curls tickling at his nose. Apparently Sherlock had decided that a warm body was much better than the pillow he'd been clutching. John attempted to extricate himself without waking Sherlock – after all, he needed the sleep even more than John did – but the heavy press of Sherlock's body made it next to impossible. As soon as John started wriggling away in earnest, Sherlock sighed in his sleep and clutched John closer.

Well, breakfast could wait, John supposed. He didn't have work today. He didn't really have anywhere he needed to be. And more sleep might not be a bad idea. Besides, Sherlock was surprisingly comfortable. John had always expected him to be as sharp, bony, and frigid as his tongue, cheekbones, and piercing stare. But Sherlock was warm, and his skin was soft and smooth, and his tummy had just the hint of a cushion now that Sherlock was eating John's toast and Mrs. Hudson's mince pies on a regular basis.

As much as John shouldn't be enjoying this, shouldn't enjoy cuddling with his flatmate or sharing his bed, Sherlock was warm, and John was sleepy, and so he wrapped his arms around Sherlock and went back to sleep.

When he woke up again, he was alone in the bed. He half wondered if he'd dreamed the whole thing. When he padded downstairs, Sherlock was eating dry toast and typing away at John's laptop, and murmured a faint response to John's sleepy "Good morning."

"Sleep well?" Sherlock asked eventually, as John stood waiting for the kettle to boil.

"Mmm," John yawned.

And nothing more was said on the matter, and so John pushed it out of his mind.