When Dean sleeps, he looks younger. His expression gains openness and credence that no one can expect to see when he is awake.
When Dean sleeps, he often keeps his mouth half open. He never gasps for air though, his lips remain sharp and steady like he is about to shut them tight, forming tiny dimples on his cheeks. He does so when he feels uneasy or worried, revealing one of these pieces of his face language that Castiel has learned over the years. But in his sleep Dean is peaceful and relaxed and never shows any of those.
When Dean sleeps, Castiel always knows which dream he has. He winces painfully and rolls on his bed at the nightmares and just smiles lightly on something pleasant, but Castiel never allows himself to find out the cause of either. Dean's dreams are sacred to him.
When Dean sleeps, the freckles on his nose bridge and cheeks are spread like stars in the night sky, and in his thoughts Castiel traces the constellations with his little finger. He never stops before he finds all the existing ones and those vanished over many millennia he has been watching them. His imaginary Lady in the Chair is on Dean's left cheek, and Southern Cross on the right, and he doesn't really care that in the sky they are much further apart.
When Dean sleeps, he is like a little star himself, distant and bright and full of inner light and generous to share it. He always shines to Castiel without even being aware of it.
When Dean sleeps, he doesn't like to be watched (and he has made it crystal clear), so Castiel never touches him and even does not hover on him to keep his breath away.
One night, he is very surprised when Dean opens his eyes and glances back.
"I don't mind," he says, and Castiel suddenly feels that this refers to something more than just watching.