You Go to Sleep on Your Own And it Scares You Being Alone (Imagine Dragons)
Cecil lay on his back with his arms stretched out to the sides, one falling off of the side of the bed, the other lying flat across the space where his boyfriend sleeps every night.
Except Carlos wasn't there. Because he was working late tonight. And now Cecil couldn't sleep.
So here he was, at almost three in the morning, waiting for Carlos to come home so he could fall asleep for at least a few hours before he had to get back to the radio station.
As Cecil grabbed and unlocked his phone, contemplating calling Carlos and saying that he had just been woken up by Khoshekh, he heard a key opening the front door to their apartment, and the door being opened followed by the soft thump of Carlos's science bag being dropped onto the floor presumably right next to the coat rack where he is now presumably hanging up his lab coat.
Cecil quickly replaced his phone on the nightstand, and pulled up the covers turning towards Carlos's side of the bed, away from the door. He focused on slowing his breathing, only closing his eyes when he heard Carlos open their bedroom door. He heard the door close, but he didn't hear footsteps for a few seconds. When Carlos did start walking again, it wasn't to the side of the bed that Cecil thought it would be; the footsteps were over near his own nightstand.
"Cecil?" Carlos whispered as he placed his hand on his boyfriend's shoulder. "Why are awake so late, and why are you pretending to be asleep?"
Cecil opened his eyes, furrowed his brow in confusion, and slowly turned to face Carlos.
"How did. . ." Cecil started, but then saw that Carlos was holding his phone. The one that he forgot to lock when he had deposited it on his nightstand. The one that was set to auto-lock after two minutes because Cecil had a habit of running down the battery by forgetting to lock his phone. Like he had just done. "Oh."
"Cecil, what's wrong?" Carlos asked, concerning running thickly through his sweet voice. "Are you having trouble sleeping?"
"Um, yeah, I have been, I guess," Cecil confessed.
"Have been?" Carlos asked. "Like, for a while?"
"Yeah, sort of," Cecil conceded. "I just have a hard time falling asleep when you aren't here."
"Oh." Carlos looked concerned. "I'm sorry, sweetie. I'll make sure not to come home this late again, okay?"
Cecil's eyes widened as he sat up straight, almost hitting Carlos's chin with his forehead. "No, Carlos! There's no need for you to miss out on doing science! I can take care of myself when you're not here, really, I can," he tried to remedy, but Carlos cut him off by grabbing both of his frantic hands.
"Cecil!" Carlos exclaimed quietly. "I know that you can, but I'm not missing out on anything except being here, sleeping with you." He shut his mouth and flustered at his word choice, but Cecil just chuckled and kissed his nose.
"Are you sure?" Cecil asked.
"Yes," Carlos told him with a kiss to his lips. "Come on," he said once they pulled away. "Let's get some sleep."
Carlos laid down, and Cecil curled up next to his side with one arm around Carlos's chest, and Carlos's arm around Cecil's back. And they fell asleep.
Person A of your OTP can't sleep and/or is an insomniac. The only way they can fall asleep is when Person B is next to them.