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Sherlock was seven when the first bout of winter insomnia hit him, and it would persist for the rest of his life. There was always something dreadfully wrong with his bedroom at night. Most of the time it was the old heater, gasping incessantly like a dying grasshopper. Or it would be the winds lingering outside his window, carrying screams and laughter from places far over the fence but nowhere at all. And at the breaking of every gray dawn, there would be bulky cleaning trucks yelping on the streets down below, sucking up all his ability to sleep along with the dead leaves.
It was also then that Sherlock developed a habit of sneaking into Mycroft's bed at night, because happy though as he was to do away with the concept of wasting time in a bed, eventually something had to give. Mycroft's bedroom was far too clean and orderly for his liking, but it wasn't as if he spent much waking time in this place. Mycroft's book shelves didn't interest him enough to stick around—The Decline and Fall of the Roman Empire, The Stranger, On the Genealogy of Morals, et cetera et cetera, and not one single volume on firearms or the properties of different poisons. (Years later, he would realize that this was not true—Mycroft simply had books on those subjects placed in more discreet places.)
If Mycroft thought Sherlock's presence was odd or bothersome, he never said anything. Thus Sherlock was free to tiptoe into his brother's bedroom—which was never locked—at the dead of the night, plant himself right under the covers, and shortly succumb to the annoying necessity of sleep, surrounded by everything Mycroft.
The smell in that bed changed over the years. Tobacco's spiciness came and went, as did the faint sweetness of butterscotch biscuit, but the odour of ink and old books remained. Eventually, right before Mycroft left for Oxford, there was the scent of aftershave, something as alien to Sherlock as Christmas carolling.
None of that, however, really mattered. Though Sherlock was preternaturally averse to all noises during those winters, the single thing in Mycroft's room that calmed him like magic was the sound of his brother's slow but steady breathing, luring him to sleep and to dream.
Fin