The first time Sherlock and John slept together was not after their first kiss, or after their first disastrous attempt at a "proper date", or first time they had sex. It was much, much, earlier on than that.

The first time they slept together was the night of the pool incident. The night that a strange twist of fate (in the form of a woman in a wisp of lace and a slash of red lipstick) interfered with a psychopath (in the form of a madman in an outlandishly expensive suit). The night that it was, thankfully, the wrong day to die.

Neither of them were willing to admit it, but even John could see that the incident had shaken Sherlock. Whether it was being at the whims of an insane criminal mastermind, the lack of control, or the sudden realisation that Sherlock cared about someone else's safety more than his own, they could both tell he was upset.

So when, halfway through the night, he showed up in John's room, bundled in his duvet, of course John just nodded and pulled back the corner of his own comforter. Of course he snuggled over to the far side of the bed, making room. What John was not expecting though, was to discover so early on that Sherlock slept in the buff.