Dean had collapsed into his mattress in the bunker with a loud thud that night, too exhausted from the wendigo hunt they'd went on earlier to even bother to pull the sheets up. Cas stayed up watching television with Sam until he begun getting drowsy and wound up heading to bed around ten or eleven, Cas was so sleepy he hardly had the energy to spend keeping tabs on the time. He wasn't used to the feeling of weariness, or being human at all for that matter. He'd just have to learn to keep up, but he'd be a liar if he said he wasn't struggling to adapt. Still, he clung desperately to his former habits as a celestial being rather than a small unassuming mortal. Old habits tend to die hard, unfortunately. He liked watching over Dean in the nighttime whilst he slept. The guilt and regret for the hurt he'd inflicted over the years weighed down on him to the point where sometimes, it was nearly too much to bear. Knowing he was able to keep Dean safe and sound at night eased his pain, and he'd always watch over Sam as well, but with Dean it was different. If he left him unguarded and vulnerable for even a minute without being there to strike down any possible attacker he found it difficult to make it through the next day. He had something different with Dean, a bond, a promise to be kept. Since he'd been cast down from heaven and was stripped of his grace, he was confined to this strange concept; the boundaries of human exhaustion. So, given no other choice, he slept on a regular basis. He tried to avoid it but quickly learned he couldn't outrun what could never be whole again, and sooner rather than later failed that task. During the chilly nights in the back of the gas-n-sip he tossed and turned restlessly in his sleeping bag, feeling the smooth concrete like a slab of ice on his back as he fidgeted. It wasn't that he wasn't tired, he was exhausted. Dean wouldn't let him sleep. As he laid there, Dean pushed and shoved his way to the front of his mind incessantly. He kept thinking about him til the very moment he dozed off; Where is Dean sleeping right now? Is he at war with the horrors in his skull and the memories of the hell he's experienced both in purgatory, perdition itself and the cusp of hell on earth his soul is earmarked for? Or is he dreaming peacefully and safely? Is there a gruff faced man with a sharp blade budging against the stubble on his neck? Either way, I should be there right now.
So during his first night back at the bunker, he hauled a wooden bar-stool into Dean's room and sat on it, watching and observing his surrounding constantly for intruders, angel blade gripped tightly by sweaty anxious hand. He stayed for hours, occasionally rubbing vigorously hard circles in his tired eyes in a feeble attempt to keep his heavy eyelids from falling. Approaching three in the morning, he could no longer stand to fight off his ever growing drowsiness and need for rest. He softly dozed off, subsequently toppling to the floor, taking the stool with him in a messy and seemingly deafening crash. Dean was on his feet in what felt like a fleeting millesecond to Cas and a lifetime to Dean, weapon gripped furiously in his hand. His ashy brown hair stuck up and out in multitudes of strange shapes and directions, and his face was terrified yet stony and hard like it was during combat as his eyes skirted about for his possible intruder, only to find Cas staring up at him dumb-founded on the ground with puppy dog eyes and it all melted away to a soft mild irritation. "Damn it, Cas, what the hell are you doin'?!" He groaned, his voice thick with grogginess. "I-I was trying to watch over you, Dean," Cas started, but Dean cut him off, "Oh Cas, come on man, you're human now and you can't keep doin' that or you're gonna run yourself into the ground. You need your rest." He said, his voice conveying a strange blend of emotions Cas was utterly unfamiliar to. "No, Dean, I need to keep you safe. I'll be alright." Cas insisted. "Shhhh, c'mere." Dean sighed softly. He came closer, still disorientated. Cas felt like he was in a daze and could hardly fathom what was happening around him as Dean sleepily wrapped his arms around Cas and guided him into his bed and yanked the covers upwards around them. "...Dean?" Cas mumbled in confusion. "Hey, Sammy ain't here is he?" Dean smiled a crooked smile before gently pressing his lips to the top of Castiel's head. "Get some rest." He shut his eyes and leaned back onto his pillow, slowly drifting off. Cas didn't think what they had meant what it meant to him to Dean, and he was in genuine disbelief. He thought surely it was a mere break from the nightmares, a sweet dream to make the rest of them somewhat bearable. He pinched himself repeatedly, and couldn't stop grinning once he elected he was still lucid and in reality, and in the arms of Dean Winchester. He certainly didn't take Dean's instructions on getting some rest, but he made up for every smile he didn't crack in weeks.