Arthur sat up in bed, leaning on his elbow and looking at Merlin; he rarely woke so early, so rarely got to observe Merlin's sleep. Now, though, was no time for dilly-dallying. "Hey, Merlin?"
He watched Merlin's nose scrunch up and settle once more. Arthur called his name again, and again Merlin's face twitched.
"Merlin?"
More scrunching. Merlin made a noise that sounded like a sneeze.
"Merlin?"
"Huh?" His eyes still closed, Merlin raised his sleep-tousled head in the general direction Arthur's voice was coming from. "Whudda'ya'wan'?"
"Just had a thought, Merlin."
"Did you?"
"Yes."
"Alright." Turning around, Merlin sank back onto the mattress.
Before his breathing could completely even out, Arthur grabbed his shoulder and forced him to sit up. "Pay attention to me, will you?"
"Arthur…" Merlin forced an eye open. "I know that thinking is a new concept for you, but for me, a full night of comfortable sleep is fairly new as well."
"Merlin."
"Arthur."
"Right." Arthur sat for a minute, trying to remember what he'd wanted to say. "Right. Sorry that you're up, but now that you're up: do you know we're soulmates?"
"…that's what you wanted to tell me?"
"Well, yes."
Merlin sighed and rubbed a hand through his hair. "What exactly brought on the realization?"
"I love you and you love me." Arthur beamed. "Simple as that."
Merlin reached up and gently kissed the bottom of Arthur's jaw before lying back down. Pulling the blanket back around himself, he reminded, "We're literal soulmates, Dollophead; two sides of the same coin and all that rot."
By the time Arthur had heard the end of the sentence, Merlin was already asleep, subtly cuddling into his side.
(So I wanted to write a soulmate!AU, then I had a realization... thanks to Rachel for betaing!)