AN: Found a video like this on tumblr and this is the headcanon that went with it. I'm also going the Zep headcanon. Not an AU, so Castiel is still an angel.
Castiel still hadn't gotten the hang of sleeping, but often grew tired when Dean did, when the sky started to darken. The days he did manage to sleep through the night with Dean, he woke up feeling odd, as though he'd missed something important, but refreshed.
Castiel rolled over against Dean in the king bed they shared. He snaked his arm around Dean's waist and sighed heavily into the back of the hunters shoulder. Dean muttered something unintelligible and unconsciously leaned back into Cas. It was about two o'clock in the morning and while drowsy, he couldn't seem to drift off. One more try, he thought, as he closed his eyes and willed himself to go to sleep.
"Daaaaaadddyyyyyy!" Zeppelin wailed through the baby monitor.
Dean groaned and went to cover his head with his pillow. Castiel reached over and pulled up the corner of the pillow so Dean could hear him properly.
"Your daughter is awake." he informed the hunter sarcastically.
"Before morning, she's your daughter." Dean mumbled before yanking the corner of the pillow back down over his ears, "Besides, it's your turn anyways."
"Paaaapaaa!" Zep wailed again through the monitor.
Castiel sighed and threw the covers off and sat up, searching blindly for sweats to pull on. Finding them soon enough and pulling them on, he staggered out of their bedroom and made his way down the hallway to Zeppelin's room at the end.
Opening the door to the bright blue room, illuminated by the moonlight streaming through the windows, he found Dean and his daughter standing upright at the bars to her crib, shaking, with tears in her eyes.
"There, there my little star, what's wrong?" asked Castiel, striding across the room to her crib, and lowering himself to her eye level. He hated when she cried, her blue eyes went stormy and dark, like a thunderstorm brewed inside them.
She sniffed and whimpered for a moment before stretching out a tiny, chubby hand to grasp at his nightshirt.
"Nightmares again, little star?' asked Cas, standing up and pulling his daughter out of her crib to wipe the tears running down her rosy cheeks. She nodded into his shoulder and slowly began to calm down.
The demon attack shortly after he and Dean adopted Zep had been quickly solved, but not before Zep had been surrounded by vicious black-eyed people leering at her and hissing threats she barely understood. It was enough to scare any child. The nightmares had become less frequent, but more intense as the months passed; she rarely went back to sleep after having one, but Castiel always tried.
"It's alright Zeppelin, papa has to go back to bed now, and so do you." Cas murmured softly to his daughter, who pulled her head from his shoulder to face him.
"Papa?" she questioned, titling her head to the side, something Dean was positive she'd picked up from seeing Cas do it so often.
"Back to bed, little star." Castiel said, dragging his daughter from his shoulder to put her back in her crib. She had stopped shaking fairly quickly, so Cas assumed the nightmare hadn't been much worse than a regular nightmare. He stroked her cheek when she looked up at him from the crib and held her hand when she grabbed for it.
"Goodnight Zep." Castiel murmured softly, leaning down to kiss his daughter on the top of her head.
Castiel loosed his hand from hers and turned to go back to bed.
"Paaaapaa." Zep called from her crib.
Turning around once more, Castiel saw his daughter was standing again, hand gripping the crib bars, another tear rolling down her cheek. He sighed; so it was going to be one of those nights.
He walked back to the crib and pushed Zeppelin to one corner of the crib before stepping into the crib with her. He lay down and got as comfortable as he could in the small space. He pulled Zep on top of him and made sure she was comfortable.
She curled up in the crook of his arm, cheek resting against his collar bone. Her breathing slowed considerably and he looked up at him with her big stormy eyes.
"Papa sing?" she asked.
"Papa sing." he replied, smiling down at his daughter.
And so Castiel began to sing;
Hey jude, don't make it bad.
Take a sad song and make it better.
Remember to let her into your heart,
Then you can start to make it better.
Hey jude, don't be afraid.
You were made to go out and get her.
The minute you let her under your skin,
Then you begin to make it better.
And anytime you feel the pain, hey jude, refrain,
Don't carry the world upon your shoulders.
For well you know that it's a fool who plays it cool
By making his world a little colder.
Hey jude, don't let me down.
You have found her, now go and get her.
Remember to let her into your heart,
Then you can start to make it better.
So let it out and let it in, hey jude, begin,
You're waiting for someone to perform with.
And don't you know that it's just you, hey jude, you'll do,
The movement you need is on your shoulder.
Hey jude, don't make it bad.
Take a sad song and make it better.
Remember to let her under your skin,
Then you'll begin to make it
Better better better better better better, oh.
Na na na na na ,na na na, hey jude...