People assume they're shagging. Even Harry, even, upon occasion, Lestrade, assumes they go to bed together. And it's almost true, except that it's really only nights when Sherlock's high-risk, or John's buzzing with undeniable tension and would probably scream himself hoarse if left to a full REM cycle. But even then, one stays up to hover over the other, who barely sleeps, and they lay in the bed, side by side, both tense with the possibilities.
When it's a danger night, sometimes, John will wrap his fingers around Sherlock's wrist, gently, a 'you are here" of sorts, a reassurance that he has somewhere to turn, finger resting against the point where his radius meets his carpals, where his pulse slams through his veins, telling them both he is alive.
When it's a tension night, Sherlock will perch on the edge of the bed, against the pillows, legs drawn up. And if, at some point, as so often happens, John dozes off, Sherlock will fetch his beloved violin and play a quiet melody, to soothe him into a restful sleep.
They don't talk about it. It's not something that needs talking about. John dates women, and Sherlock scares them off, and they fight crimes and argue about Sherlock using medical gauze as a potholder when John asks him to get the tea off the stove and he's too lazy to open the drawer and fetch a proper one. But the one time John started to, Sherlock interrupted to say "Ninety percent of human communication is performed via body language, John, and there was no objection in either yours or mine. So why would we need to discuss it?"
And that was that. When they needed too, because there was a bed too few or one of them needs it, they sleep beside each other, and when the night is over, they get up and continue with their lives.