As an angel, Castiel never had to deal with sleep and so far he's not seeing the appeal of it. He preferred the sleepless nights where he could look around at all of God's creations and think of all the good that's going on in the world. However, since becoming human again the nightmares have started to return.
The mistakes he's made that have cost people their god damn lives, and he wonders how Dean and Sam ever forgave him… or if they ever really did.
That's the worst part. The uncertainty.
Sometimes he wonders whether he should just leave, go back to heaven or just live a low profile until no-one cares anymore, but he's selfish and can't bring himself to leave the life he's made for himself.
He admires Dean because despite everything he's been through he still manages to have hope that one day things will get better. He wishes he still had those aspirations but he couldn't lie to himself the way everybody else seems to be doing.
So instead of sleeping Castiel just lies there, staring at the ceiling where the damp has just started to form and thinks; thinks of all the good times, all the bad and wishes he could fly away.
Tonight was particularly bad.
He woke up to Dean shaking his shoulders softly and whispering nothings into his ear, trying to get him to calm down and stop the screams that he didn't realise were coming from his throat .
Afterwards, he was still shaking and clutching Dean with all his might but he felt better because at least he knew he had someone.
He told Dean of how he felt to blame for so many deaths and described the pain in their eyes and he killed them. Every last one.
And no matter how much Dean tells him that 'it wasn't like that' and 'I know you never wanted to hurt anybody' the guilt was still there and it was killing him.
Dean stayed with him that night, just holding him and it was the first time he'd properly slept in months.