While Dean was drifting off into sleep, Cas was alert and attentive, paying careful attention to Dean's heartrate, his breathing. He lay quiet and still, so as not to disturb Dean; these early stages of sleep were easily disrupted, and he didn't want to startle Dean awake. Dean needed his rest. He steadied his vessel's functions, trying to match the rhythm of Dean's body as closely as he could. He listened to Dean's breaths, noting the way they deepened, becoming louder now that he was not more conscious of them. His metabolism would be slowing now as well, his blood pressure decreasing. His brainwaves changed, lengthening and slowing.
Cas wondered what it was like to truly feel these human body functions, if it felt like time slowed down when your body did. He didn't really remember falling asleep the few times he had - angels didn't sleep naturally; he had just been too drained at those times to maintain his vessel, so it took care of its own needs. Dean seemed to enjoy it anyway, especially here.
He'd watched over Dean before, countless times. There was something different about how he slept now, relaxed and almost vulnerable. Dean didn't push himself to utter exhaustion here. There was no need to, and Castiel was glad that Dean had managed to find at least some peace here - though he did still keep a gun under his pillow, Cas noted with fondness. Dean had never invited him to share his space while he slept before, not this closely. Dean hadn't even liked him to stand guard over him before. The depth of trust it showed was utterly humbling. For a while, Castiel just lay there, wrapped up in his favorite human, practically glowing with contentment. He was unable to even feel guilty about it, for once. Dean wanted him here; that was enough.
Dean shifted slightly, and Cas shifted too, pulling back just a little to watch his face as he slipped into REM sleep. Dean's eyes started roaming under the lids, not with the urgency of a nightmare, but gently, as with a good dream. Cas smiled, just enjoying Dean's body heat and the soft flutter of his eyelashes. Dean's hand was still pressed against his back, warm and possessive and welcome.
Lying close like this, he would have been content just to watch Dean sleep all night. On the other hand, he was curious what Dean was dreaming about, and he had been invited. Cas sighed softly and closed his eyes, nuzzling back into Dean's hair. He focused inwards, letting his consciousness slip beyond the confines of his vessel and probing at Dean's mind. It was almost easy to slide into Dean's dreams, made open by lack of conscious awareness and more active brainwaves. Everything was waves, in the end, true angelic forms far more similar to human consciousness than anyone wanted to admit. Curious, that.
Dean seemed aware of his presence already, too, his dream shifting as though to make room, the other figures disappearing. Cas made sure to give himself a shape more familiar to his friend, his hands tucked into the pockets of his overcoat. He smiled softly at Dean. "Well, you did say I could join you," Cas reminded him. He looked around, admiring the dream-saloon that Dean had conjured up. "This is nice. Not historically accurate, of course, but it's your dream."