The first time it happens, Sherlock is annoyed. They’re traveling by taxi. The crime has been solved, dinner has been had, and Sherlock is expounding on the final crucial mistake the criminal made. His pontification is disrupted by a single soft snore from the seat next to him. He looks over to see John Watson—his conductor of light, his best friend, his whatever-they-are-to-each-other, and possibly the only person on the planet who understands him—leaning against the window glass and drooling slightly.
Sherlock sighs and folds his arms into a sulk the rest of the way home.
The second time, he’s less annoyed and more curious. What is it about taxis that John finds so soothing? This case only lasted for roughly 51 hours, hardly enough to warrant complete exhaustion. And Sherlock knows that John managed to sleep for 18 minutes while they were at the Yard awaiting the arrest warrant. He turns his attention to the problem of the sleeping flatmate.
For the remainder of the ride to Baker Street, Sherlock watches John sleep as if trying to solve a puzzle.
The third time, John doesn’t fall towards the taxi window. One moment, Sherlock is talking about the case (a daylight robbery this time—it should have been much easier to solve than it was, but for a few dull and ordinary details anyone could have missed), the next his words stop mid-vowel as John’s head gently thumps against his arm. Sherlock sits very still, and looks down at him. There’s a bruised scrape across one of John’s cheekbones where he got into a scuffle with the robber—John really should have put ice on it, it’s going to swell terribly by morning.
John is warm and heavy, muscles limp with exhausted sleep. It’s disturbing at first, like seeing a puppet with its strings cut. John looks too vulnerable. Anything could happen to him like this. He wouldn’t be able to defend himself. Something about that thought tightens Sherlock’s jaw. Moving with infinite slowness, he slides his arm out from under John’s head, and lets him fall against the side of his chest, near his heart. He lowers his arm around John’s shoulders and turns to rest his chin atop John’s head. John smells like soap and wool and home.
After a moment, Sherlock closes his eyes.