Castiel moved fast. He has been watching for exactly those signs. He was onto the bed in a moment, his hand gripping Sam's shoulder, the other on Sam's forehead. "Sam! Sam open your eyes. You're safe. It's a dream Sam. Only a dream." He pressed his hand to Sam's head firmly, holding him steady.
He had hoped Sam would have one night's unbroken sleep but that was yet to happen on his watch. He leaned in close, his voice urgent and low, all his attention on Sam. "It's aright Sam. I am with you." If he didn't wake Cas would have to go in.