Sherlock fell asleep when he was moving.

Okay, not literally, John reasoned, but Sherlock fell asleep every single time they got into a form of transportation. If it moved, it didn't matter what it was.

One case had ended at three-thirty in the morning, in Cardiff, and they were on their way back to Baker Street when Sherlock had conked out. Sherlock was in the front with Lestrade, John was in the backseat (because, apparently, Sherlock rode shotgun if he had the option and John was too tired to argue with him). Lestrade and Sherlock were talking about the latest details from a case - how Sherlock kept going in moments like this, John had no idea - when, abruptly, there wasn't an answer to one of Lestrade's questions.

"Sherlock?"

Still no response and John perked up slightly, trying to focus on the conversation. If Sherlock was ignoring Greg again, for whatever reason, it was John's duty to step in and talk.

"He fell asleep," Lestrade said.

"... What?" John mumbled, flicking his gaze to the detective even though he couldn't see his face.

"He was just talking and he... he's asleep," Lestrade said, humour evident in his voice. "Well, that's a fine tip-off to how boring I am."

John laughed quietly, resting his head against the car window.

Another case had taken them out of town by train. It always happened when they went out of town, because there was an extended wait to get back to Baker Street and into their beds. So, when they had a fifty minute train ride - not to mention checking in afterwards... The outcome should have been (but yet wasn't) obvious.

John left the carriage for all of five minutes to drag his tired and bruised body to the loo. When he came back, Sherlock was sprawled out across the seats, his arm dangling with fingers brushing the floor, snoring quietly. His coat had been discarded and had fallen to the floor. John was also vaguely aware of the fact that Sherlock's shoelace was untied.

With a sigh, he grabbed Sherlock's coat, shook it out, and draped it over Sherlock's body. He never got enough sleep.

(Of course, this didn't stop him laughing when Sherlock tripped over the forgotten shoelace at King's Cross.)

Another time, a case led them to America. Seven hours into a nine and a half hour plane ride to the Midwest, Sherlock's body clicked off whether his mind wanted to or not. He'd been going two and a half days without a wink of sleep over the rigorous trail they'd been following. It was enough to tucker even the most enthusiastic consulting detective out.

Sherlock's head flopped over onto John's shoulder and made him jump.

"Sherlock?"

John tilted his head sideways, looking at Sherlock's face.

"Sherlock."

When he was sure that Sherlock was still breathing, that he had just fallen asleep without any precursor, John sighed. "Sherlock..."

Despite the strange looks they got, John didn't prompt him to move. Instead, he just put his ear buds back in and returned his attention to the movie playing currently.

So, Sherlock fell asleep when he was moving. He literally never stopped. It was kind of a scary thought, really. John? All he asked for was a cosy bed and a warm hearth...

... although it didn't stop him falling asleep on the sofa one night, ending up sprawled halfway across Sherlock's lap by the time he woke up, and all through the spluttering and the embarrassment and the "why didn't you wake me up?!"s, all Sherlock did was look up from his book and say "I didn't need to get up".

/raises hand I can sleep in the back of a car. Very comfortably, I might add. But they'll be an actual chapter for vehicle sleeping. I wanted to focus on different types of transportation here. Car's coming soon.

Thanks!