Sherlock slept naked.
John knew this. He wished that he didn't.
Really, it had all started innocently enough. Sherlock had fallen asleep on the couch after a long night out, apparently.
John had been out with Stamford the past three hours. He hadn't seen Sherlock for almost thirty-six hours- but he had sent him a text to say that he was alive a day ago, so John stopped worrying- but when he had returned from his night-out, Sherlock had been passed out on the sofa, a bit worse for the wear.
"Sherlock? Sherlock, wake up. Come on... you smell," John said dryly.
Sherlock moaned and rolled over, or tried to. Being on the sofa didn't allow for much movement room and he ended up just edging further into the back of the couch.
"Sherlock, go have a shower and go to bed. And then I'm going to get you a once over in the morning," he added, noting the large discolouration of skin just peeking out from his shirt on his shoulder. "And you're going to tell me where you got that bruise."
Sherlock pried his eyes open, staring blearily towards the back of the couch. "... Joh...?"
John sighed. "Yes... Yes, I'm back. So are you, apparently. Go have a shower and go to bed."
Sherlock struggled to roll over. "How was..." he trailed off, eyelids fluttering closed.
"It was fine," John clarified, taking Sherlock's arm again. "Up you get."
Sherlock glanced up again and, after probably more trouble than it was worth, John managed to get the lethargic detective to the bathroom. He stopped up the bath and ran in the water, leaving Sherlock to manage the rest of it on his own.
When the water drained and Sherlock didn't resurface, John just assumed that Sherlock had gone back to his own bedroom and didn't bother to check on him.
However, by the time that John woke up at ten the next morning and Sherlock still wasn't to be found, his bedroom door still closed, John felt justified in going to check on him.
What he was found was Sherlock, but a very naked Sherlock.
He was sprawled out in bed, completely naked, face-down and cuddling his pillow. He hadn't bothered with the duvet or sheet, or, if he had, he had lost it in turning over in the middle of the night. His hair was a mess. He looked thinner than usual, and John accidentally noticed a freckle that Sherlock had on his upper backside.
John closed his eyes and counted to ten, letting out a deep breath. He strode across the room and grabbed the blanket, drawing it up to Sherlock's shoulders without once looking away from the wall.
After checking to make sure that the consulting detective was still alive and breathing, John turned and strode out.
Sometimes, no, scratch that, most of the times, John wondered why he was constantly forced to be Sherlock's babysitter.
Although, a little voice reminded, you wouldn't be happy without him.
Really, that was the scariest thought of all.
Because, given that Sherlock walks out of his bedroom in A Scandal in Belgravia wearing nothing but a sheet, we can infer that he sleeps naked. :) Unless he was just randomly in bed without clothes on. Whatever. :p