Based on a Sherlock headcanon on Pinterest
John had never really minded Sherlock's violin. Most of the time, he played beautiful melodies that filled the whole flat.
He did, however, mind when Sherlock began to play at odd hours in the morning. He would groan and bury his head under a pillow until it ceased. But otherwise, it was lovely.
John glanced wearily at the clock. "Sherlock, it's already midnight. Want to take a break, maybe eat something?"
"How many times, John? I don't eat when I'm working."
"It's unhealthy," he replied.
They were working on a complex case involving an old widow, a murder, and a susipious flower seed packet.
They worked late into the night. John did eventually buy something from the cafe downstairs and force some down Sherlock's throat. He ended up falling asleep on the couch.
Sherlock rubbed his temple and sighed. This case really was puzzling. He glanced at his friend, frowning when he noticed him shifting in his sleep. He knew John had nightmares from his time in Afghanistan; not that he'd told him. Sherlock had deduced it himself.
John's brow creased and he muttered something inaudible, clenching his fist.
Sherlock watched him, toss and turn, fearing he might fall off the couch. He wanted to wake John up, but it might seem creepy that he'd just been staring at his friend the whole time. He turned back to his work before noticing his violin out of the corner of his eye.
His violin.
Sherlock quickly picked it up and began to play a melancholy tune he made up as he went along. John groaned, sitting up, breathing fast. "Sherlock, for once in your life, could you just be quiet so I can sleep?" he asked. He was secretly a little thankful Sherlock had saved him from his nightmare ridden slumber, but didn't let it show.
"Glad to see you're awake," Sherlock remarked wryly. John mumbled back, and lay back down.
Sherlock smiled faintly and began to write down the piece of music he'd just played, titling it John's Lament.