Every night, for as long as anyone could remember, Sherlock Holmes would put down his book and get ready for bed. And every night, on the dot without fail, Mycroft would turn of the lights with a sigh and a fond smile. And every night, when the lights were out and the world still, Sherlock would slip into his brother's room and sleep sound until the sun rose.
Mycroft didn't mind at first, but as the months turned into years he began to worry. They couldn't carry on this way for ever. But try as he might, the brilliant mind of Mycroft Holmes couldn't come up with a single solution that didn't involve talking to the child directly, so that's what he did.
Sherlock was in the library when Mycroft found him, knees drawn up to his chest, a torn copy of Treasure Island clutched in his small fingers, pale blue eyes zooming across the page drinking in every word. His messy black hair was falling in front of his eyes, it was a wonder the boy could see.
Mycroft walked over to him and muttered "What's the matter Sherlock?" The child in question looked annoyed at being interrupted but put the book down at the look on his brother's face, well more the look in his eyes. His faced softened and he just stared back. As Mycroft stood in silence next to his little brother he felt a rush of affection for the boy, this feeling was short lived however as it was quickly replaced by the front, the mask he always wore, even to Sherlock, even to himself. "What's wrong?" he asked again, slightly more firm. When Sherlock didn't answer he began to get angry. "Sherlock answer me! You can't even sleep in a room on your own what's wrong with you?!" There was still no sound from the ball of silk that would one day be the great Sherlock Holmes." Fine, fine. You won't talk. I should've expected as much. Fine, you're sleeping on your own tonight and that's that!"
He began to walk away, when he heard a small voice from behind him" Okay Mycroft, you win, I'll talk." Mycroft went towards the 5 year old and knelt down so they were at eye level. Sherlock began to explain "It's just, every night, when the lights are off and everyone else is asleep , I find myself staring into the darkness and seeing things. My mind is inventing things and it's terrifying. I hate it, but I can't stop it." He broke off, worn out now. Mycroft stared out the window in deep thought for a moment before standing up suddenly. "Come with me."
They walked up to Mycroft's bedroom. He disappeared in side for a minute or two before returning with something in his hands. A record. Bach. He gave it to his younger brother whilst saying "Now, dear Sherlock, listen to this every night as you go to sleep, don't think, just listen."
So that is why John Watson can be found being driven from his bed in the wee hours of the morning, every morning, by the sound of a single violin. It is being played not just to clear his head and help him think, not just to turn on the lights and make the sun rise, but to scare away the nightmares.