Castiel waited until the Winchesters were asleep before appearing in their cheap motel room. Sam was snoring lightly in his bed, but he was not the reason Castiel was there.
At the moment the older Winchester seemed to be sleeping peacefully but the angel knew it wouldn't remain that way. Once Dean reached what humans called rem sleep he would be haunted, once again, by his time in the pit. So Castiel waited.
The angel didn't have to wait long. Soon Dean's muscles tensed and his hands began clenching and unclenching the bed sheets. He tossed and turned fitfully in the bed, his eyes tightly closed, his brow furrowed, and his jaw clenched as if in agony.
Castiel went to his side. He gazed down at Dean with an uncharacteristically soft expression. He cared for this human.
The angel placed his hand on the top of the hunter's head, healing Dean's scarred soul as much as he could that night. Castiel smiled as Dean let go of his sheets, his jaw unclenching and his muscles loosening. He lay still, no wrinkles marring his forehead, a small smile on his face. For a brief moment Castiel hoped he was dreaming of nice things.
Just like he did every night Castiel waited until he was sure Dean's nightmares were gone for the night. When he was sure they were gone, he vanished, knowing full well he would return the next night, and the one after that, to keep Dean's nightmares away as well as he could.