A drabble written in response to this .gif set on Tumblr.
Those few who were acquainted with Sherlock knew he didn't sleep much, assuming that he spent those hours when most normal human being slept working on his latest experiment or case. Maybe that had been true in the past, but with the entry of John into his life that had changed. Now Sherlock spent a number of hours each night watching John sleep, learning the things very few ever took the time to notice because for some unfathomable reason, he could never get enough of the study of John. Even during his waking hours Sherlock spent far too much time partaking in his favorite hobby, until John caught him staring for too long, and Sherlock went back to pretending he wasn't watching him.
Sherlock hadn't quite understood what it had meant, his fascination, his inability to get enough of John, at least not until it was far too late. Seeing John so broken because of him, Sherlock could never forgive himself for that, but there was no alternative in this, at least not one Sherlock would have ever considered. He knew the type of man Moriarty had been and the breadth of the web he had stretched across the world.
If Sherlock happened to spend a few extra hours sleeping each night, he did so because at least in his dreams he could be with John. It wasn't the same, not nearly, but they and the memories were all he had.