The noises coming from the sitting room are loud and discordant. John is reminded unpleasantly of someone strangling a cat. Sherlock's irritated then. Stretching, John scratches his stomach, debating the relative merits of crossing Sherlock on his path to the kitchen versus food and coffee. Eventually food and coffee wins. It always does.
He trudges down the stairs and peeks his rumpled head into the sitting room, where Sherlock is flapping about in a deep red dressing gown, violin resting loosely on his shoulder while he gesticulates with the bow. It takes John a moment to realise he's writing with it - big loopy trails in the air. Sherlock turns towards where John is standing and stares right through him for a moment, focusing on something only he can see. Not angry then - concentrating. It's a relief.
With a slow blink, Sherlock comes back to earth and looks strangely anxious for a moment.
"John, when did you come down? Did I wake you?"
"What? No, no... It's fine." he mumbles, but is betrayed by a rather dramatic yawn.
With a small smile, Sherlock once again puts bow to string, but this time the music is clearly for John's benefit, not his own. John pours himself some coffee as the flat is filled with the familiar, comforting strains of the Beatles.