Sherlock sleeps in and went to sleep late, John went to sleep at a normal hour and wakes up early. This wasn't a problem before they slept in separate beds every night, but more recently it's been, sort of, just annoying. Sherlock lost out of hours of John time each night, and John lost out on Sherlock time each morning.
Sherlock always went to bed late because he liked the night better. That was no secret. He liked to be alone at night and bask in the moon, rather than the sun. He liked the sounds of sleeping London and the light of London night lights. He liked hearing John's sleeping breaths and feeling his warmth. Sherlock wished he could sleep when John slept, to be awake when John was awake, but his body and mind just could not do it.
John would wake up around 6 AM, usually just out of habit. He had to be very tired to sleep past 6 AM. He'd wake up and turn over to see Sherlock's sleeping face right next to his own pillow. He'd smile.
"Good morning, my love."
Sherlock could hear him, he could hear everything in his sleep. He'd wrap his arms around John's waist and pull John close.
"No, Sherlock, I've got to take a shower."
Sherlock pulled tighter.
"Fine, twenty minutes."
John might have fallen asleep for an hour.
"Ok, I have to get up, I need to shower."
"John," Sherlock whispered, "What would it take to keep you in bed until I wake up?"
"You going to sleep when I do."
"No, I mean now."
"I don't know, you'd have to persuade me."
Sherlock rolled on top of him and instantly stuck his tongue in John's mouth.
"How's that?" Sherlock asked, gasping for air.
"Are you still asleep?"
"I might be." Sherlock kissed John again while pulling John's underwear down to his knees.
They stayed in bed for another two hours, and after that Sherlock knew how to keep John in bed.