John took at least seventeen minutes in the bathroom in the morning to get ready for work, between showering, shaving, brushing his teeth. That gave Sherlock two minutes to make sure his flat mate was semi-permanently enclosed in the room, one minute to remake the bed after, one minute to rearrange himself downstairs in an innocent pose, and thirteen minutes in the middle to curl up in the warm spot in John's bed.
Sometimes he spread out, deducing John's sleeping position by the limits of the warmth on the on the bottom sheet. Other days he just curled up and let the man's warmth seep into him.
If John ever noticed that his bed wasn't made with quite the same military precision as before he went to shower, or if he noticed a stray dark hair on his pillowcase, he never said. He might see, but he never observed.
Sherlock tried waiting once until John left for work before invading his bed so he might luxuriate the whole day, but John took too long and the bed was cold by the time he got there. Instead, he spent an hour imagining John sharing the pillow, his face and warm breath so close. It wasn't difficult. John's scent, clean soap, wool, and a warm, woodsy smell when he lit their fireplace, lingered on the pillow and scent was a very strong memory aid.
One morning, John left for the bathroom and Sherlock counted out the two minutes in his head. The shower turned on and Sherlock leapt up the stairs on cat feet. He'd no sooner pulled the covers back than someone cleared his throat.
John cleared his throat.
Directly behind him.
"May I ask what you're doing in my room?" His tone was full of good humor and capped with a small amount of exasperation.
"Laundry, John," Sherlock lied. "I need to see the effects of bleach on common cotton sheets."
"No. Use your own sheets. I won't have you ruining mine."
"But mine are 1600 threat count…"
"No, Sherlock," John said more firmly.
Sherlock made a show of tossing the edge of the blanket back over the side of the bed and made no fuss to ensure it was tidy.
"Oh, very well, John. I'll ask Mrs. Hudson for one I can use for my experiment." He swanned out the door with no little relief that John believed him.
Then, after a long case where John hadn't worked at the clinic much and Sherlock hadn't time to sleep or eat for days, much less steal into John's bed for his thirteen minutes of comfort, Sherlock paced in the sitting room, unable to quiet himself. John had admonished him to go to bed, though he had retreated to his own with the belief he'd wake to Sherlock sprawled, passed out on the sofa or possibly the floor. Sherlock could feel his transport begging for sleep – the feeling gnawed at him, clutched at his eyelids and shorted out his brain. He thought about fighting it longer, just to prove his will was in control, not his body.
But there was another sensation in his body, one he couldn't quite identify. He'd felt it before. When? It took an improbably long moment for Sherlock to place it. Right. He felt it just before sneaking into John's bed. The need for warmth. Cold, he was cold!
Sherlock leapt up the stairs two at a time, rapped twice before opening John's door. John raised his head blearily from his pillow.
"What is it, Sherlock?"
"I'm cold, John," Sherlock announced with no small amount of pride in his discovery. "Budge over."
John was too tired to argue and if this was what it took to let Sherlock settle, to let him sleep, to let John sleep, then so be it. The second he shifted to a cool section of the mattress, Sherlock's gangly limbs were tucked into the warm spot.
"Oh, yes, perfect," Sherlock sighed and laid his head on the pillow with its warm indent from John's tousled head. John fussed with the covers, tucking them about Sherlock's neck. Then he settled in, head bent close to Sherlock's as the detective had appropriated his only pillow. They had to share unless John wanted to crawl out of his warm comfortable bed for another one.